A rare phenomenon called red algae bloom has briefly closed Sydney's Bondi Beach to swimmers as the crimson tide came close to shore.

Also closed was the nearby Clovelly Beach while authorities conducted tests on the area in and around the Tasman Sea.

Swimmers were immediately advised to steer clear of the moving algae due to potentially harmful reactions.

Also known as noctiluca scintillans or sea sparkle, the algae has not been proven to have toxic effects for humans - but people have been advised to avoid contact as it is high in ammonia which can cause skin irritation.

"It has got quite a fishy smell to it," local lifeguard Bruce Hopkins told the Australian Associated Press. "It can irritate some people's skin but generally not much more than that."

The closure of the beach will be a disappointment for the locals, British tourists and expats who are basking in Australia's summer temperatures of up to 40C.