Lake Burley Griffin is closed because of the highest blue-green algae readings on record, 1000 times above safe levels.
Blue-green algae levels tested at more than 66 million cells per millilitre yesterday, compared with the 64,000 cells per millilitre earlier this month.
A National Capital Authority spokeswoman said this was the highest level on record.
ACT Health said extreme exposure to the contaminated water may lead to death.
The Captain Cook Memorial Jet in Central Basin was also turned off because the winds can create fine mists and cause jet water to travel around the lake, potentially exposing the public to algae either through direct contact with or inhalation of the mist.
The authority spokeswoman said the rise in algae was created by the ''autumn bloom'', where water surface temperatures cool, creating a mix of the water profile and lifting nutrients from the sediment and promoting algae.
Recent low winds and low rain have created the right conditions for algae growth.
Water quality results received yesterday indicated blue-green algae levels were 1000 times higher than the level considered safe for activities such as swimming.
The authority has banned swimming and whole-body water contact activities at the lake. Secondary contact such as rowing, fishing and boating is still permitted, however, people are being urged to take extreme caution. People have also been warned to keep their pets out of the water.
Authority chief executive Gary Rake said the Anabaena circinalis strain of blue-green algae was more dangerous than the summer strain.
''Falling into cold water, and being exposed to high levels of the Anabaena algae could prove fatal,'' he said.
ACT Health advised the Anabaena circinalis strain produced hepatotoxins and neurotoxins.
''Hepatotoxins attack the liver and other internal organs, and may cause visual disturbances, gastroenteritis, nausea, vomiting and muscle weakness,'' a statement from ACT Health said.
''Neurotoxins act as neuromuscular blocking agents.
''High levels of exposure can result in death by paralysis.''
ACT Health has warned people to avoid rowing, canoeing, kayaking and boating, unless they are experienced, and to not directly touch the water.