The algae that turned some of Sydney's beaches red this week is making the ocean glow blue at night.
The striking blue light was spotted at Bondi and Coogee beaches late on Wednesday night, as well as south of Sydney.
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Sydney's glowing tide
The crashing waves of Bondi and Coogee caught onlookers by surprise as blue light shone from the water, a product of the recent algae infestation.
Laurence Palmer was leaving dinner with his girlfriend at Bondi when he looked out to sea and noticed the glow.
"The most amazing blue light could be seen coming from the ocean. People were standing and staring at the incredible natural light show," he said. "When the waves crashed the sea was turning blue."
A marine microbiologist from University of Technology Sydney, Justin Seymour, said the algae glows when the water is agitated, leaving a trail of neon blue.
"Typically what makes it light up is the water becoming disturbed from movement and turbulence," he said.
It occurs when there is a high concentration of algae in the water, like the red algal bloom that coloured the water at Bondi Beach earlier this week.
"You often get these blooms occurring when you have a combination of warm sunny weather following periods where you might have had some rain flushing nutrients into the water," Dr Seymour said.
He says there is debate within the scientific community as to why the bioluminescence occurs.
"There's some evidence that they might produce it as a defence mechanism against predators."
The natural light show has been stunning onlookers at Bondi for the last three nights. Bruce Hopkins, the lifeguard co-ordinator for Waverley Council, said it will probably be visible tonight at Bronte and Clovelly, where some algae is still close to shore.
Bioluminescence is a tourist attraction in parts of Puerto Rico, where people swim in "bio-bays" and watch the water glow around them.
I've never seen anything like it in my life. It was like something from space.
The natural phenomenon was not contained to the eastern suburbs beaches this week. Matt Smith spent a few hours on Wednesday night watching and photographing the light at Stanwell Park Beach, south of Sydney, where he lives.
"I looked out the window and I could see really vivid blue lights flashing up and down the beach," he said.
Initially he thought it was a fisherman with a high-powered light.
"The streaks of light were so bright, like lightning underwater illuminating the breaking waves."
He also compared it to flashing police lights.
"I've never seen anything like it in my life," he said. "It was like something from space."
The algal blooms are continuing to close beaches along the NSW coast, with sightings as far south as Bateman's Bay and as far north as Newcastle.
Bilgola, Mona Vale, Palm Beach and Newport on the northern beaches were all closed on Thursday, as were Clovelly, Bronte and Tamarama in the eastern suburbs.
A spokesman for the NSW Office of Water said it was difficult to predict if the warmer weather forecast for the weekend would increase bloom outbreaks.
''It's really up to mother nature and the current weather and ocean conditions prevailing,'' he said.
A spokeswoman for Waverley Council said the 12th annual Bondi to Bronte ocean swim would go ahead on Sunday as planned. However a final call will be made at 7.30am, with updates available on the council Facebook page.