The lake was hit with the bloom after being cut off from the River Murray by a newly constructed levee to protect properties in the town.

Almost a fortnight ago, authorities said only part of Lake Bonney was affected by algae and the levee would be removed as soon as possible after the river peaked, allowing for natural water flows to resume.

However, testing on Thursday revealed potentially harmful levels of blue green algae, prompting a warning from health experts for people to avoid swimming and diving at the lake until further notice.

Up until the most recent testing, parts of the lake – especially around the jetty – were deemed safe to swim in.

People could still go boating, fishing, canoeing and rowing, but were advised to take extra care not to ingest the water, SA Health said on Friday.

Those who had direct contact with the water could see their skin, eyes, ears, nose and mouth irritated, and swallowing algae-affected water could lead to illness.

Symptoms could include abdominal cramps, diarrhoea, vomiting and nausea.

Pets were particularly vulnerable to becoming unwell because of the algae, which could stick to their fur, SA Health said.

-AAP