TasWater's involvement in the Prosser Plains Raw Water Scheme

What is the Prosser Plains Raw Water Scheme?

This is a Glamorgan Spring Bay Council project to supply raw water from the Lower Prosser Dam to Louisville Point and beyond, via a series of pipes and pumps. The water will come from the proposed Twamley Dam or Hobbs Lagoons, and will only have access to the amount of water released from these sources. The Prosser Plains Raw Water Scheme will not use treated water from the Orford Water Treatment Plant.

TasWater is working with Council on this project to ensure the security of the local drinking water supply. All questions about the Prosser Plains Raw Water Scheme should be directed to the Glamorgan Spring Bay Council.

What is the Hobbs Lagoon proposal?

Just as the Prosser Plains Raw Water Scheme proposes to transfer water into the Lower Prosser Dam from the yet-to-be-constructed Twamley Dam, the Hobbs Lagoons chain near Stonehenge has also been proposed as an alternative source. This would operate under the same conditions as that of the Twamley Dam, with transmission losses being accounted for as it flows along the Bluff River and water also being made available for use by TasWater’s customers.

What is the Twamley Dam?

The Prosser Plains Raw Water Scheme includes a proposed dam on the Tea Tree Rivulet, within the property known as Twamley. This is a Glamorgan Spring Bay Council Project, not a TasWater Project. If approved however, the Twamley Dam could form part of the overall solution to the area’s bulk drinking water supply. Council has allocated TasWater 200 megalitres from either the Twamley Dam or Hobbs Lagoons for the Orford Water Treatment Plant, enough to supply Orford for a year. 

Does TasWater support the Twamley Dam proposal?

TasWater is providing in-principle support for the project and is working with all parties to ensure the long-term sustainability and efficiency of this dam, and that it is operated in an equitable manner.

Initially, TasWater was concerned about potential transmission losses of water along the Prosser River during flow from the proposed Twamley Dam and TasWater’s Lower Prosser Dam. Those initial concerns are being addressed, to ensure no detrimental impacts on the reliability or quality of TasWater’s supply.

In addition, Glamorgan Spring Bay Council agreed to allocate 200 megalitres of water to the Lower Prosser Dam for use as drinking water, if the Twamley Dam is approved and built. As TasWater’s only concern is the security and safety of the water supply, this back-up allocation provides additional security for our customers.

What assurance is there that TasWater will be able to access water from a privately owned dam?

The 200 megalitres has been secured through a legal agreement which does not include any financial element.

What is ‘transmission loss’?

When water flows from one water source to another, it follows a water course. River or creek beds contain holes and voids which water can disappear into. After a drought, water can also be absorbed by dry ground. Evaporation is another factor that creates water loss, along with vegetation along the river. All of this means that when water is released upstream, less water arrives downstream.

Calculating the amount of transmission loss is an important part of managing the transfer of water. Once rates of transmission loss are calculated, they are used to ensure that the amount of water needed in the Lower Prosser Dam is achieved by releasing a larger amount upstream, from either the Twamley Dam or Hobbs Lagoons.

Won’t building a new dam just block the river further upstream?

New dams are not designed to completely block the flow of a river. Modern dam standards require 50 per cent of river flows to be released for environmental purposes.  Any new dam will more effectively capture large seasonal rainfall events, so that water can be used at a later date. TasWater has a good understanding of just how variable the rainfall can be on the East Coast and how it is changing over time. We are confident that there is enough water in the Prosser catchment for all users, providing these sources are managed responsibly.

The Summer 2018 Water Restrictions

Why did Orford and Triabunna go onto stage 1 water restrictions April 2018?

Between January 2018 and May 2018, water was not flowing into TasWater’s Lower Prosser Dam, the primary drinking water dam for the area. This coincided with a period of higher temperatures and increases in population during the holiday season creating an increase in water demand. This caused dam levels to decrease. 

How can there be a drought when it’s raining in Orford?

Rainfall in the town of Orford has no bearing on the water flowing into the Prosser River’s Dams. The maps below from the Bureau of Meteorology show the size of the catchment and the location of the rainfall gauges (please note that rainfall on Maria Island has no bearing on this discussion).

TasWater relies on rainfall data from gauges near the major tributaries into the Prosser River, namely:
  • Bluff River in Woodsdale (Stalworth)
  • Brushy Plains Rivulet in Runnymede
  • Tea Tree Rivulet in Nugent Twilight Valley
  • Upper Prosser River in Levendale.
It should also be noted that rain in the catchment does not always produce flow into the dams. It is not a 1:1 relationship. Depending on the amount of rain, the temperature, the dryness of the ground, evaporation and transmission loss, much of the water will not be available for use as drinking water.

Did Tassal cause the 2018 water restrictions in Orford and Triabunna?

No. We understand some customers are concerned that a private company may have impacted the resilience of Orford’s water supply. We have made it very clear that this was not the case.

During January 2018, Tassal experienced operational issues with their desalination facilities and temporarily drew water through the Orford reticulation system to fill fish bathing liners. TasWater detected this usage at the time. It came from an existing connection that does not normally draw these volumes. We contacted Tassal to request they stop as soon as possible, which they did.

How much water did Tassal use in January 2018?

TasWater is legally not permitted to provide information on any customer’s water use. However, Tassal has publically acknowledged it used approximately four megalitres of water in January 2018. In the context of the low rainfall and high temperatures at that time, it is possible that Tassal’s increased usage created the need for water restrictions approximately a few days earlier than otherwise would have occurred anyway.

Can TasWater provide the data to prove its statements?

Yes, we’ve provided the following graphs:

  • Demand for drinking water from the Orford Water Treatment Plant, July 2014-May 2018
  • Prosser catchment rainfall and the Lower Prosser Dam level, July 2015-May 2018
  • Inflows into the Lower Prosser Dam and dam level, July 2015-May 2018

We advise caution when attempting to interpret this data. Climate and hydrological data are affected by many variables, and therefore can be complex to interpret. For example, seasonal temperature increases demand, but this varies greatly from year to year. Data from a wet February will be different to data from a warm February in the previous year. Also, rainfall does not always produce flow into dams. It is not a 1:1 relationship. Depending on the amount of rain, the temperature, the dryness of the ground, evaporation and transmission loss, much of the water will not be available for drinking. TasWater also bases some of its operational decision making on weather forecasting, which can change the data but is not captured in it.

Demand for drinking water from the Orford Water Treatment Plant, July 2014-May 2018

This graph shows the variation in demand between winter and summer, as the temperature and population increases. Though January is typically the month with the highest usage, the variability of climate from month-to-month means demand is highly variable.

Another variable is that Orford Water Treatment Plant can service both Orford and Triabunna, as required.

January 2018 is the highest level of recorded demand on the graph, approximately 31 megalitres. This is the month in which Tassal used the additional four megalitres that has been reported.

Prosser catchment rainfall and the Lower Prosser Dam level, July 2015-May 2018

You can download a larger version of this graph here.

This graph displays both the level of the Lower Prosser Dam (in blue) and the average of all the rain guages monitored by TasWater in the Prosser catchment (in red). Anytime the blue level of the dam increases above zero, the dam is spilling in response to increased riverflow as a result of rainfall in the catchment. If the dam is already full at this time, only a small amount of rain is needed to make it spill.

You can see how the level of the dam falls during periods of low flow, usually coinciding with warmer and drier weather, but also that those weather conditions can occur at different times of the year.

The data also shows that sometimes the level of the dam does not substantially increase in response to rainfall in the catchment. This will be during dry conditions when rain is absorbed by dry ground and does not restore flows to the Prosser River.

Dry conditions throughout 2017 were briefly interrupted by rainfall in December 2017, which caused a relatively small spill of the Lower Prosser Dam. However, with high summer water demands and without constant flows from the river in the following months, the dam level began to steadily drop. Water restrictions were required because the dam level dropped significantly further than in previous years. This did not happen suddenly in response to a single event. It took weeks.

Note there is a period in early 2016 when no data is available for the dam level.

Inflows into the Lower Prosser Dam and dam level, July 2015-May 2018

You can download a larger version of this graph here.

This graph is similar to the one above, showing both the level of the Lower Prosser Dam (in red) and inflows into the dam (in blue), and the periods of water restrictions. It is important to understand that the flow data is an average of daily flows. This means the data may ‘hide’ both larger inflows and a lack of flow, because following rainfall events flows can vary greatly throughout the day.

The inflow is measured from a flow meter between the Upper Prosser Dam and the Lower Prosser Dam, so this indicates anytime the Upper Prosser is spilling, or where there are contributions from streams and creeks that join the Prosser between the two dams.

Again, the graph shows that a relatively dry 2017 was briefly interrupted in December 2017, and the lack of water flowing into the Lower Prosser Dam in the early part of 2018, which created the need for water restrictions.

Why did TasWater give Tassal water?

We didn’t give Tassal any water. Tassal is a customer that must pay for any water it uses.

Why did TasWater allow Tassal to exceed their water allocation?

Tassal does not have an allocation. They pay for what they use, like any customer.

Why didn’t TasWater stop Tassal from taking the water?

TasWater has no legislative power to stop any customer from using drinking water they are paying for. As part of TasWater’s overall management of a water source, if one customer’s usage is found to be negatively impacting the larger network, then TasWater does have the power to disconnect a customer. It is TasWater’s preference not to do this however, and this would only take place as a last resort. In this case, after detecting the increase in drinking water use, we asked Tassal to stop and they did. 

Do water restrictions apply to Tassal?

Yes. It should be noted that the nature of stage 1 water restrictions are relatively minor. They are designed only to minimise unnecessary outdoor water consumption by approximately 20 per cent, impacting watering of gardens, sports grounds and washing of vehicles.

What interaction does TasWater have with Tassal?

As a large customer with many sites around Tasmania, TasWater has a customer service manager who handles our relationship with Tassal. We also provide assistance with the removal of waste.

How was the Freestone Point Road development assessed for its impact on the local water supply?

In general, after any Development Application (DA) is submitted to the relevant local council, TasWater is given the DA for consideration where appropriate. TasWater assesses the DA’s stated requirements against hydraulic models of the water and sewerage systems. TasWater then issues a Submission to Planning Authority Notice (SPAN) which sets out any conditions or restrictions. The SPAN then forms part of the Planning Permit issued by council.

For the upcoming Freestone Point Road development by Tassal and Seafish Tasmania, TasWater’s SPAN specifically states that drinking water must not be used to bathe fish. In April 2018, the Tasmanian Planning Commission released its assessment of the rezoning of Freestone Point Road, specifying that the developer is to adhere to TasWater’s requirements once the Freestone Point Road development proceeds, and TasWater expects this will be the case.

It should be noted that the events of January 2018 are not covered by this SPAN, as they involved an already existing connection, which does not normally draw the amounts of water recorded during January 2018.

How will TasWater ensure Tassal do not continue to use drinking water to bathe fish?

TasWater does not have direct control over what its customers choose to do with the water they pay for. 

TasWater's operations

What is the Orford Triabunna Water and Sewerage Strategy?

TasWater has a long-term strategy to prioritise our investment in the area, to ensure the ongoing delivery of reliable water and sewerage services for the coming decades.

Orford and Triabunna’s water supplies can be vulnerable to drought, which makes water restrictions more likely. However, significant rainfalls can also be experienced. The towns’ existing four dams are not big enough for the reliable supply of water during drought, but if large rain events are captured in larger storages, there will be more than enough water for our customers well into the future.

Regarding sewerage services, the Orford and Triabunna sewage treatment lagoons are currently too small to meet expected growth. Public health and environmental outcomes will be improved through planned upgrades to the sewerage networks and treatment plants.

Given that TasWater faces similar issues throughout the state and does not have the budget to fix everything at once, we take a strategic approach to the gradual upgrading of infrastructure as required. Any water or sewerage projects we deploy will have already been identified within the strategy, so they can form part of a broader solution to the area.

What are TasWater’s plans for Orford and Triabunna?

As part of TasWater’s strategic approach to Orford and Triabunna’s water and sewerage, a range of project proposals have been identified. All proposals are given nominal dates for implementation, but do not yet have funding. It is to be expected that scheduling will change, based on priorities which may emerge from further investigation or as needs dictate.

Given that it is not possible to simply fix everything at once, TasWater can ready projects as needed, knowing they will form part of a larger strategy that is designed to minimize overall costs and improve efficiencies.

Already complete

  • Orford and Triabunna Water Treatment Plants upgrades – ensuring their continued performance and minimising the risk of failure, ultimately providing the time to plan and fund a replacement plant for the area.
  • Decommissioning of Holkham Sewage Pumping Station – which was built prior to TasWater’s ownership in a wetland area, was subject to flooding and presented a risk to the public.
  • Orford Sewerage Outfall Project – repairing and extending the outfall to provide better dispersion of treated effluent, and improved health and environmental outcomes.

Looking ahead

  • Orford to Triabunna Trunk Main Stage 1 - TasWater’s long-term strategy calls for a new treated water main along the Tasman Highway, to improve the transfer of water between the two towns. In partnership with Glamorgan Spring Bay Council, this pipeline is expected to be built in 2019, and installed in the same trench as pipelines for local property development and industry. Each partner is funding its own part of these works, but the collaboration will reduce overall costs and disruption to the area.
  • Upgrade of Triabunna reservoir – This upgrade of the treated water reservoir was originally scheduled for 2016, but has since been deferred. It would not compensate for shortage of raw water in dry periods, it is intended to improve water pressure in higher parts of Triabunna and help balance the daily demand.
  • Upper Prosser Dam expansion – TasWater has conducted an investigation of the area to confirm that an expansion of the Upper Prosser Dam could provide a solution to the area’s lack of bulk water storage. However, if the Glamorgan Spring Bay Council’s Twamley Dam proceeds, then the need to construct additional storage at the Upper Prosser Dam will not arise until further into the future. The size of the additional storage capacity required at Upper Prosser Dam and the timing for its construction are currently under review.

Further details of these project proposals and others will be provided as they develop.

How much water does Orford and Triabunna use?

Demand for water changes in response to a number of variables, so the exact amount changes each year. On average, the two towns use around 271 megalitres (ML) per year. The Orford Water Treatment Plant also provides some water to Triabunna when needed, so the typical demand is best explained with this diagram.

What are the four dams that service Orford and Triabunna?


  • Bradys Dam - 70 megalitres
  • Maclains Dam - 5 megalitres


  • Lower Prosser Dam - 225 megalitres
  • Upper Prosser Dam - 220 megalitres

The above numbers are considered maximum possible storage available, but do not represent the final amount of raw water available to TasWater customers, since that is subject to weather, transmission loss, condition of the network and other factors.

These dams are managed by TasWater, which includes regular inspections. There are known issues associated with each dam, but they are all largely working as designed and intended, with no evidence of significant leakage.

See the FAQ below for further details on each dam.

Does TasWater account for growth in Orford and Triabunna in its management of the water and sewerage?

Yes. Australian Bureau of Statistics data is used to anticipate growth, in local population, shack owners and tourism. We also assess local development information, employment trends, usage data, tourism data and more.

TasWater blames water restrictions on droughts, but isn’t it TasWater’s fault for not doing its job properly?

Orford and Triabunna’s water systems are vulnerable to drought because they do not have enough bulk water storage. Building a new dam is an expensive exercise to create infrastructure that will possibly be in use for over 100 years and it takes time to get it right. TasWater must prioritise work on infrastructure throughout the state, but we accept there is community frustration when events such as pipe breaks or a temporary increase in usage by a customer appear to lead to water restrictions.

The actual amount of water lost in these events is relatively minor, but if it occurs when water restrictions are already likely, it’s more of a problem than during times when the storages are full. The loss of four megalitres from a small storage with a dwindling supply is more of a problem than the loss of four megalitres from a larger supply.

You can read more about our planning for a new dam in the area here, in a list of proposed future projects for the area.

Can TasWater provide its hydraulic model of the Orford Triabunna water system?

TasWater uses a computer-based model on a software platform called Infoworks WS which requires training to use and understand. The calculations within the model inform TasWater’s strategies. The information contained within the model is quite detailed and is normally kept confidential, which enables TasWater to maintain its authority over development decisions. So while it is not possible to “provide” the hydraulic model, TasWater is able to discuss some of the detail with interested parties.

Why have the floodgates on the Lower Prosser Dam never been fixed?

TasWater staff are often asked about the broken hydraulic floodgates which were damaged in 1969. The floodgates were meant to capture additional flows for storage during a flood, although this additional storage would only be minimal. Given that the floodgates buckled and failed during their first use, it would appear they were not designed or built to the necessary standards.

Thus far, TasWater has chosen not to allocate funding to repair the floodgates. To completely replace the floodgates would be an expensive project for little additional storage. Their repair would not negate the need to invest in a new dam in the area.

Are there problems with the Upper Prosser Dam?

It was intended that during a drought, water from the Upper Prosser Dam could be released along the river into the Lower Prosser Dam, via a manually operated outlet valve. Unfortunately, the valve was poorly designed and has since been deemed unsafe to operate. Any staff operating the valve would likely be underwater.

As an alternative, TasWater can install a siphon to bring water over the dam wall. In fact, we were in the process of planning this in May 2018 when heavy rain ended water restrictions for the area.

Are there problems with Brady’s Dam?

There can sometimes be water quality issues for Brady’s Dam, with low oxygen levels during cooler weather making it unsuitable for drinking. In this case, or during planned maintenance, Bradys Dam is withdrawn from service and Triabunna is temporarily supplied from the Orford Water Treatment Plant.

Are there problems with Maclains Dam?

Only that it is too small to provide a significant benefit to the area’s water supply.

Why can’t TasWater just fix all the dams now?

The four dams which service Orford and Triabunna are on a list of more than 300 dams throughout the state which TasWater manages. Repairs to any dam can be complicated by poor access for staff and heavy equipment, safety concerns, working near full water storages that can spill at short notice and requiring divers.

At this stage, all four dams are working largely as intended. Although there are some relatively minor issues, there is no evidence of significant water loss. TasWater must carefully consider the costs and benefits before choosing to invest in capital works.