The National Capital Authority banned swimmers on Friday from many areas of Lake Burley Griffin due to the high levels of algae present in the water. It joined Lake Tuggeranong on the off-limits list.

The algae blooms have been caused by a potentially record-breaking heat wave across the ACT, with at least nine days of temperatures more than three degrees above the monthly maximum average.

Meteorologist Ben McBurney said the warm weather was caused by a persistent high pressure system which has sat off the NSW coast for a number of weeks, bringing high temperatures and low winds.

"We've had nine consecutive days above 18 degrees now," he said. "The previous record for consecutive days this late in the season was nine days and that was in 1947. This one could go up to 12 days or even further than that."

He said there was a possibility of cooler temperatures coming through on Friday, but more than likely the heat would continue well into next week.

"The winds have been very light, with no dominant winds at all, making for very pleasant days. It's been stunning weather," he said.

But a spokeswoman for the NCA said the blooms were one unfortunate consequence of the unseasonably warm autumn temperatures.

“Unusually warm days with little wind, coinciding with cold night-time temperatures, creates deeper mid-lake water to rise to the surface. This warm water flows to the lake edge bringing with it the blue-green algae,” she said.

“The bloom will last until day-time temperatures drop which will stop algal growth.”

West, central and east basins were closed when the high algae levels were detected and, shortly afterwards, Yarralumla Beach, Yarralumla Bay and Weston Park East were also closed.

People have been banned from any activity that involves being in the water, including swimming, waterskiing and windsurfing. Boat use is still permitted.

The algae can potentially cause flu-like symptoms, gastrointestinal illness and skin irritation and pets can become very sick or even die from drinking water with high levels of algae.

NCA chief executive Malcolm Snow said anyone who used Lake Burley Griffin should be cautious.

"If contact is made, users should ensure that any algal material is removed from the skin by showering immediately after exiting the water," Mr Snow said.

Territory and Municipal Services closed Lake Tuggeranong in March after "extreme" levels were detected in the water.

It remains off limits to the public until further notice.