Authorities have slapped an emergency alert on Lake Tuggeranong, saying the waterway presents an ''extreme risk'' to the health of swimmers and animals.
The Environment Directorate has placed the lake on its highest risk category of extreme, saying blue-green algae levels at the popular water spot have reached dangerous levels. There have been no reports of any lake users suffering health problems, but in a statement a spokeswoman for the Environment and Sustainable Development said the decision had been made following advice from ACT Health.
The directorate has banned ''primary contact recreation'' until further notice.
This includes any recreation activity that involves the body or head being submerged in water, including swimming, diving or waterskiing.
While rowing, fishing and boating will still be allowed, lake users are being warned to avoid any contact with the lake unless they know the risks of algae, and wash their clothes thoroughly after any contact.
''In particular, contact with algal scum should be avoided,'' the spokeswoman said.
''Clothing that has been in the water should be washed to remove any blue-green algae residue.
''Pet owners should not allow their animals to swim in or drink the water, as blue-green algae can make pets very ill or even result in death.''
Blue-green algae occurs naturally in Australia, both in fresh water and sea water, erupting into ''blooms'' during droughts and heatwaves, or with high levels of nutrients in the water.
According to the directorate, blue-green algae release toxins into the water as they die, which can cause skin irritations and gastroenteritis.
The Molonglo Ski Area remains closed after briefly re-opening in November, after a sandbar moved in the area, which the directorate said might put public safety at risk.