Tests carried out by Griffith City Council last week found potentially dangerously high levels of the bacteria in the north lake.

All recreational activities such as swimming, boating, fishing and yabbying have been banned in order to prevent residents coming into direct contact with the water.

Council general manager Peter Brooks said the closure was a precautionary measure due to the high algae levels and continuing warm weather.

"It's in the best interests of the community that we close the lake now to ensure there are no immediate health hazards to people who would normally participate in water sports," Mr Brooks said.

The news is a blow for the lake's recreational users, including the Griffith Sailing Club.

"Luckily we just had our big event on the weekend but we sail every Sunday and we've got an increasing number of juniors being interested in sailing," the club's publicity officer John Brickhill said.

"So this is going to put a major halt on our program."

The algae will be monitored weekly and the lake will remain closed until levels meet acceptable standards.