Hydro Tasmania needs to stop draining one of the state's biggest lakes to feed one of its power plants, a leading Tasmanian environmental campaigner says.

Great Lake is currently at 11.4 per cent capacity, and has fallen 2.5 metres below its September 2015 mark.

Hydro is still feeding water from the lake to the Poatina Inlet power station as the state deals with a power crisis.

The lake is currently classified as a high environmental risk zone prompting environmentalists to urge Hydro to stop draining the lake.

Tasmanian Conservation Trust director Peter McGlone said wildlife in the area was at risk.

"Hydro really have some questions to answer," he said.

"At what point do they get beyond the high-risk category?

"They've hit the high-risk band and they're continuing to take water every day."

Great Lake feeds Hydro Tasmania's Poatina power station which was first commissioned in 1966. ( ABC News: Kieran Jones )

Earlier this week, Hydro said it was using water from Great Lake sparingly because of environmental concerns.

But Mr McGlone raised concerns about the impact of the low dam levels on fish in Tasmania, arguing the mitigation measures may be too little, too late.

"It's really important that the level of Great Lake doesn't get drawn down too low, not only because of the direct effect on the fish but because we risk destroying the algal beds they rely on," he said.

"There does seem to be some platitudes in Hydro's statements around the importance of this lake and the algal beds.

"We have a fundamental problem with how lakes are managed in Tasmania.

"Even if there's an agreed lake level there's no law currently that forces Hydro to stick to that."