The Metropolitan Sydney, South Coast and Hunter Regional Algal Co-ordinating Committees yesterday said they had warned "communities along coastal beaches between Sydney and the Central Coast about algal blooms.
"This bloom has likely occurred as a result of the upwelling of nutrient-rich deep ocean water on to the continental shelf," they said.
"Testing of samples taken from Bondi beach have been identified as noctiluca scintillans, which appears as a pinkish to reddish discolouration in water.""Upwellings create food sources for the predatory noctiluca scintillans - there's more bacteria and phytoplankton for them to eat," Dr Murray said.
"They live on the surface off the coast and are very buoyant, so they are at the mercy of wind and ocean currents. If you have the right mix they can be concentrated in dense groups and pushed on to the beaches.
"These two algal concentrations - on the northern and eastern beaches - would have come from the same offshore source.
"They aren't toxic but accumulate ammonia because of what they eat, which can be irritating to some people."