Water testing under way after sewage flows into Wollongong Harbour

The pollution forced Saturday's Wollongong Splash to be shifted to North Wollongong beach, amid fears of high e-coli levels, and meant freedivers couldn't enter the water during Clean Up Australia Day efforts.

A Sydney Water spokesman said a blockage on Kembla Street was responsible for a "wastewater overflow" on February 27.

"Crews responded quickly to clear the blockage … removing tree roots, and setting up containment measures. However, wastewater flowed into the stormwater network which led to Brighton beach," he said.

"Sydney Water has since completed a full clean-up of the blockage and stormwater."

Signs, warning against swimming, have been installed in the area.

"We are awaiting the results of water quality sampling in the coming days to confirm the water is clean, before the precautionary signage can be removed," he said.

"Sydney Water apologises for any inconvenience this may cause to swimmers and appreciates their continued patience and understanding.

"Sydney Water takes its responsibility to protect public health and the environment seriously."

Meanwhile, beachgoers have been urged not to make contact with water discoloured by a marine algal bloom along the coast.

Tests have identified it as Trichodesmium erythraeum, a species not considered toxic but with potential to cause skin and eye irritation.​

Reports of the algal bloom, which currently appears as a brownish discolouration throughout the water and can be mistaken as an oil slick, have been received along the Sydney coast from Palm Beach to the Illawarra.

In later life stages, the species may be visible as a pinkish discolouration in the water.

Trichodesmium erythraeum occur in tropical and temperate environments and can move down the coast via the East Australian Current.

Authorities will continue to monitor the bloom.