The Environment Protection Authority issued an alert on Friday after finding the level of algae in the lake carried increased health risks.

Banned activities include swimming, diving, waterskiing and windsurfing.

The Environment and Planning Directorate has warned pet owners to not let their animals drink or swim in Lake Tuggeranong as Tychonema algae can kill pets or make them very ill.

Rowing, boating and canoeing are still permitted on the lake but only if participants are experienced, informed of the algae risks and what to do if contact occurs, do not submerge their body or head in the water, and have access to shower facilities with clean water.

The troubled lake is regularly closed over summer due to unhealthy algae levels and has been described as a "a cesspit" by Tuggeranong Lake Carers.

The ACT Government is trialling floating wetlands as a solution to lake's ongoing battle with blue-green algae.

In 2014 there were two closures of Lake Tuggeranong. After a 42-day closure beginning in December 2014, the lake was reopened on January 28.

The Environment and Planning Directorate said the latest closure followed advice from ACT Health and was in accordance with ACT Guidelines for Recreational Water Quality.

The directorate has urged residents to avoid touching algal scum and to wash any clothes that come into contact with the lake water.

Water event organisers should ensure participants are aware of the Tychonema algae alert level and the associated exposure risks and provide adequate shower facilities.

Anyone concerned about possible symptoms of Tychonema algae exposure should contact their GP.

For more information, phone Access Canberra on 13 22 81 or visit www.environment.act.gov.au