Randwick City Council advised would-be swimmers not to enter the water this weekend over fears over the affect the algae could have people's skin.
"There is currently an algal bloom affecting our coastline from Coogee to South Maroubra," the council said on social media.
"Algal blooms are a natural phenomenon which discolour the water and can cause skin irritation."
An algal bloom occurs due to an increase in algae and cyanobacteria in the ocean, which can lead the water changing colour from green to brown or even red.
It is also known to irritate the eyes, nose and skin if people come into contact with it.
Symptoms have been described as similar to hay fever.
Beaches affected include Coogee Beach and South Maroubra, as well as a number of rock pools in between.
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