Bondi Beach could be closed for up to 36 hours while tests are conducted on the algal bloom that stained the water a red colour this morning.
A spokeswoman for Sydney Water said a maintenance team, including a water quality expert, arrived at Bondi at 11am to carry out extensive tests on site, plus take samples to be tested in laboratories.
Bondi closed as 'red tide' moves in
Tests are being carried out on a number of beaches along Sydney's Eastern suburbs as a 'fishy' red algae turns the sea red.
''We think it is looking very likely that it is a blue-green algal bloom,'' she said.
Algal blooms often appear in hot and humid weather, such as experienced in Sydney during the past three days. An expert from the NSW Department of Water said blue-green algae is often brown, red or pink in colour because of photosynthetic pigment, which gives a brown, red or pink colour to the cells.
The Sydney Water spokeswoman said there appeared to be no issues with the sewage treatment plants at Bondi and Malabar. The managers of those plants had confirmed they were in working order, she said.
While Sydney Water was fairly confident the bloom had not been caused from pollution or sewage, it was doing tests on site to check for faecal coliforms.
She said those results would be available quickly, but the tests determining what kind of algae was causing the bloom would take as long as 36 hours. Unofficial results would likely be given to Waverley Council, which manages Bondi, within 24 hours.
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 had seen all sorts of algae, including red and blue.
While some algae has been spotted off the shore of neighbouring beaches at Tamarama and Bronte, Mr Hopkins said they were still open. Clovelly has been closed.
A volunteer at the Surf Lifesaving club in Bondi said the bloom was washing up on shore: "It's staining the sand as well."
Fairfax Media photographer Edwina Pickles said the bloom had not stopped some people from playing or swimming at Bondi.
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 the Bondi Pavilion.
The spokeswoman from the NSW Department of Water said it was impossible to tell with the naked eye whether the algal bloom was dangerous to humans.
She said some algae irritated the skin and caused eye infections.
Sydney Water’s maintenance team is also taking samples from other beaches affected by the bloom, including Clovelly, Tamarama and Bronte.