Beaches closed over health fears but some swimmers are braving the water.
Phenomenon caused when tiny plants flourish due to unusual conditions.
They can appear in various colours often with spectacular results.
Algae is high in ammonia which can cause skin rashes and eye irritation.
Large numbers of fish are believed to have perished.
Tourists heading for world-famous Bondi Beach were left high and dry today after a rare natural phenomenon turned the water blood red.
Bondi was among several popular beaches in and around Sydney, Australia, which had to be closed after a huge algae bloom transformed the sea into something resembling a scene from a Jaws movie.
But despite the warnings a number of intrepid beachgoers were seen venturing into the water and swimming through the red surface, Ten News Sydney reported.
Known as Nocturnal Scintillans or sea sparkle it has no toxic effects but 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.
Continue reading: "The crimson tide: Tourists in Australia flee as BondiBeach turns into the 'Red Sea' because of rare algae bloom">>>