Bondi Beach has reopened after the algae that turned the water red faded and disappeared from sight, leaving only a few traces in the sand.

While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation.

Sea sparkle, which resembles fairy floss, gets its name because it may appear to be phosphorescent at night.

The Metropolitan Sydney South Coast and Hunter Regional Algal Co-ordinating Committees (RACCs) attributed the algal bloom to an "upwelling of nutrient rich deep ocean water onto the continental shelf"', which was often seen after rainfall near river mouths.