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.