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.

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."