It was the third shark at the beach in four days.

The shark, which a witness said was about three metres long, was spotted by North Bondi lifesavers at about 2.55pm.

"We then got everyone out of the water and launched two jetskis to shoo it back out," a Waverley Council spokesperson said.

It is believed the shark had disappeared by the time lifesavers reached the area it had been spotted. Swimmers were allowed back in the water about 3.35pm.

Witness Karl Hayes said the shark was "a pretty big thing", about three metres in length.

"It was moving across the beach toward the north end. It surfaced quite a few times," he said.

"It was a bit of excitement, that's for sure. Everyone got out of the water very quickly."

Once allowed back in the surf, snorkellers returned to the spot where the shark had been sighted, Mr Hayes said.

Two great white sharks have been caught in the nets at Bondi Beach this week. A 2.15m shark was pulled from the net earlier on Saturday morning, while a 2.5m shark was discovered on Wednesday.

Thousands of Sydneysiders are expected to flock to beaches on Sunday, with the mercury forecast to hit 35 degrees.

Waverley Mayor Sally Betts said the community would need to be "really vigilant" in the wake of recent shark sightings at the beach.

"It's not something that makes us happy," she said.

"I was told that the other day there were quite a lot of fish in the sea at the moment. That's normally when the sharks come in, when they're following fish."

The council was trialling the use of drones to provide a live camera feed of the surf and they could be in use this summer, Cr Betts said. She reminded swimmers that the shark nets do not cover the entire width of the beach.

"I think we all understand the nets are not a permanent barrier, it's a deterrent," she said.