Lake Tuggeranong has been reopened to primary contact some six months after the government warned swimmers to stay away, or be affected by blue-green algae infestations.
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A sample from February 25 found the lake contained more than 100 times the levels of dangerous "cyanobacetria" recommended for safe swimming at 5.67 million cells a millilitre.
Measurements from April 12 indicate the cell count has dropped to 2160 a millilitre, which is below the low alert level threshold of 5000 cells.
University of Canberra researcher Dr Fiona Dyer, who is leading a project to find a long-term blue-green algae solution ahead of next summer, said the drop in cells was expected.
"This is part of the natural cycle of algae as they're very much a warm weather phenomenon. Once it starts to get cold, the algae [dies off]," Dr Dyer said.
"If you're game enough in this cold weather to jump in the lake, that's very much up to the individual. It's safe."
Lake Tuggeranong has been closed to primary contact, which includes activities such as swimming and water skiing, since December 2018.
Blue-green algae exposure can cause skin irritation, flu-like symptoms and gastrointestinal illness. At its worst, it can make neurotoxins that destroy nerve tissue and are potentially deadly to animals and humans.
The Environmental Protection Agency announced that Lake Tuggeranong had been reopened on Friday.
Dr Dyer's project was being funded as part of ACT Healthy Waterways, the bulk of which wraps up on June 30.