Canberrans have been warned against having contact with Lake Burley Griffin after government testing found elevated levels of the dangerous blue green algae and other bacteria in the water.
The National Capital Authority advised on Wednesday that several areas in the lake, including the central basin and Yarralumla Beach, had been closed to "primary contact" until further notice.
Primary contact includes activities such as swimming and bathing. "Secondary contact" activities such as sailing, canoeing, and rowing were discouraged for inexperienced lake users.
"There is an increased risk of adverse health effects from water exposure," a statement from the government body said.
"Symptoms of exposure may include skin irritation, flu-like symptoms, and gastrointestinal illness."
People who do have contact with the lake are advised to wash their skin as soon as possible afterwards with water and soap, and seek out medical help if symptoms occur.
Animals can also have adverse reactions to water with a high concentration of blue green algae, and pet owners are advised to not let their animals swim in or drink from the lake.
Toxins from the bacteria are potentially deadly to animals as they can cause liver failure.
Event organisers were also advised to ensure participants were wary of the high blue green algae levels, and that adequate showering facilities were provided if people were having contact with the water. The algae forced Canberra Triathlon Festival organisers to scrap the swim leg and re-badge Saturday's event as a "dryathlon".
The closures, which have also affected the east basin, Weston Park east, and Weston Park west in Lake Burley Griffin, are the first since the recreational season began in mid-October.
The National Capital Authority will continue to monitor lake water quality, with samples collected weekly. Canberrans are advised to look out for warning signs by the lake.
"We remind the public that by keeping gutters and drains clean of leaf waste and grass clippings, less nutrients are washed into the lake," a statement said.
"Over time green waste increases the nutrient load which often aids blue green algae growth."
Under the ACT guidelines for recreational water quality, two consecutive clear water samples must be collected before alerts can be lowered or removed, and swimmers can return to the water.
Last month, the territory government begun work on three new wetlands that it said would eventually let Canberrans return to regular swimming in Lake Burley Griffin.
The three new sites, to be built in Fyshwick, Narrabundah, and Kingston, would join an additional nine sites across the ACT that were scheduled for completion in June this year.
It would take an additional two years for the plants in the wetlands to grow tall enough for natural filtration to occur and cleaner waters to flow into Lake Burley Griffin.