Beaches at Shellharbour were closed on Wednesday after a red slick for more than 1km appeared near the shore.

Shellharbour City Council erected signs prohibiting swimming at North Shellharbour Beach, believing the substance to be noctiluca scintillans. This marine algal bloom has high ammonia levels that can irritate the skin.

Yesterday the discolouration appeared off Belmore Basin and at North Wollongong Beach (pictured), although a Wollongong City Council spokeswoman said that warning signs would not be erected as the substance was not dangerous.

She advised people to avoid swimming in the red water as a precautionary measure.

"The algal bloom is a natural occurrence for this time of year along the NSW coast.

"These blooms usually occur as a result of the upwelling of nutrient-rich deep, ocean water onto the continental shelf - but can also often be seen after rainfall events in the vicinity of river mouths," she said. "This species has no known toxic effects but may cause minor skin irritation in some people.

"The bloom off our beaches has gradually come closer to the shore due to the currents and council would like to remind the community that while no known toxic effects have been reported, people are advised to avoid swimming or surfing in discoloured water."

Last week both the Metropolitan Sydney and Hunter Regional Algal Co-ordinating Committees alerted communities between Newcastle and Sydney to the presence of the algal bloom.