The southside lake is undergoing a massive algae bloom.

University of Canberra water ecologist Ross Thompson said it was the worst he'd ever seen the lake.

"It's almost like a green enamel paint ... it's a big bloom," Mr Thompson said.

The lake has been closed to primary contact for some time since a blue-green algae outbreak, but Mr Thompson said as the nights start to get cooler, the algae will get worse.

He said in the peak of summer, the warmer top of the lake and the colder bottom helped keep the nutrients separate.

But as the warmer top of the lake started to cool, the water mixed, bringing with it the nutrients and the algae.

"It's much worse than I expected," he said.

"It's pretty nasty looking."

And the smell? "Pretty awful."

The worst of the blooms were seen around the Sea Scouts building and across from the Tuggeranong town centre.

Mr Thompson said he didn't yet know if the algae was producing toxins.

"It would be good for members of the public to keep an eye out for dead birds or fish gasping at the surface," he said.

The water ecologist is currently doing studies on the lake's water quality, hoping to find a way to finally treat the ongoing algae blooms.

"We'll get another month of work out there and then it will get too cool to look at the algae," Mr Thompson said.

But he's also pointed to the need for funding to keep up the research, saying the current funding from the ACT government's Healthy Waterways Projects was set to dry up in June.

"Ideally we'd be doing more experiments," Mr Thompson said.

The tests had already produced some promising results, he said and hoped with ongoing research that he could have clearer answers on how to treat the problem by next summer.