Hydro Tasmania is planning to spend $20 million strengthening a dam that was built on a fault line, with recent studies showing a one in 10,000 chance during any one year of an earthquake bringing it down.
Edgar Dam is one of three built around Lake Pedder, deep in Tasmania's south-west wilderness.
About 100 kilometres downstream from the dam is Huonville, a town of 3,000 people.
The dam was built in 1972 on top of the Lake Edgar fault line, but as seismology research has improved in Australia in recent decades, Hydro re-examined the risk of catastrophe.
The fault line has seen three earthquakes of at least magnitude six in the past 60,000 years, and one in the past 18,000.
The risk of another is one-in-10,000 every year.
What could happen to the dam – and the downstream communities – should another earthquake occur is not publicly known, although Hydro has shared its risk assessment with emergency services.
Restore Lake Pedder convenor Christine Milne said the public had the right to know this risk, and the documents should be made public.
"Tasmanians are not children. We don't need the Hydro to make a decision on our behalf as to whether we can cope with documents showing us the risk," she said.
"We're just getting, 'well, take it from us, it's our word for it'. Well, no, we want to know."
Flood risk modelling not released
Hydro decided the dam needed to be strengthened based on this risk.
The works involve putting more rock on the downstream side of Edgar Dam and excavating the pond to check it's on solid foundations.
Hydro Tasmania head of civil assets Andrew Hickman described the rocks as a "catch fence" for particles if an earthquake occurs.
He said the works would reduce the risk of failure of the dam – a risk that sits on a sliding scale.
"What's quite likely is if it was to fail, it happens over days and weeks and months," he said.
"Depending on the size of the ground motion from earthquake, it could happen reasonably fast, and in that instance we want to make sure we're well positioned with the community in Huonville.
"The absolute minimum duration from us being aware that there's been an earthquake, to seeing any water in Huonville as it rises, is something in the order of at least 10 hours."
The community is being asked to take Hydro at face value.
Mr Hickman said the full risk and inundation modelling was only being released to emergency services – and not the community – in order to avoid panic.
"That information out of context, wouldn't want that to unsettle people unnecessarily," he said.
Restore Lake Pedder campaign uses dam concerns to highlight cause
Underneath this discussion is the campaign to restore Lake Pedder.
Most of the people who attended a Hydro community consultation session in Huonville this week were part of this campaign.
Lake Pedder was inundated in 1972 for Tasmania's hydro-electric system, leaving the original lake well beneath the surface.
Decommissioning Edgar Dam is central to the campaign, with a proposal for a staged draining.
But plans to strengthen the dam are in direct competition.
Ms Milne said there would still be some level of risk to communities like Huonville.
"You can only strengthen a dam to a certain extent. If you have two or three metres where the ground rises one or either side of the fault line, then no amount of strengthening it is going to remove the risk," she said.
"One of the arguments for the restoration of Lake Pedder is you remove the risk to downstream communities, and you have all the benefits of a restored Pedder."
According to Hydro, the three dams that impound Lake Pedder provide about six per cent of Tasmania's energy.
Mr Hickman said the future of the dams was up to the government, not Hydro.
"Hydro Tasmania's role is to produce reliable power, and that's what this is about, reliable and safe power," he said.
The Tasmanian government has no plans to decommission the dams.
Primary industries minister Jo Palmer said they were an important part of the state's renewable energy mix.
