It is understood that Clovelly beach has also been closed, but officials haven't confirmed the closure yet. A sign on Bondi Beach said the beach was closed due to "dangerous conditions".

Bruce Hopkins, the lifeguard co-ordinator for Waverley Council, said the beach was closed early this morning after the lifeguards arrived at 6am.

As the lifeguards prepared the beach for the public, they saw algal bloom "out the back" that began to drift towards the beach.

"We decided to close the beach and warn the public," said Mr Hopkins. He said the bloom was very obvious and has "a real fishy smell".

"If you can't see it, you can definitely smell it," he said.

Mr Hopkins said he had been working for 21 years as a lifeguard and during that time he'd seen all sorts of algae, including red and blue.

He said Sydney Water tested the water about 9am, and will determine whether the beach is safe for swimming.

While some algae has been spotted off the shore of neighbouring beaches at Tamarama and Bronte, Mr Hopkins said they were still open.

Nobody was swimming but webcams showed surfers at the north end of the beach.

A volunteer at the Surf Lifesaving club said the bloom was washing up on shoreline: "It's staining the sand as well."

Fairfax Media photographer Edwina Pickles said the bloom hadn't stopped some people from playing or swimming.

She spotted four swimmers and six surfers and a handful of children building a sandcastle near the water's edge.

"There are also a whole lot of tourists taking photos of the algae," she said.

Most of the bloom is on the sand and sea outside of the Bondi Pavilion. While the algae is staining the water, it isn't visible in the sand.