Sydney Harbour turns brown as pollution warning issued for most NSW beaches

People are being told to avoid swimming at most beaches along the NSW coast due to pollution.

The storms and floods have caused the water in Sydney Harbour to turn brown and many of the ocean beaches are filled with debris.

Pollution is "likely" at all Sydney beaches from Palm Beach right down to Cronulla, the state's environment department has warned.

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"Due to extreme wet weather conditions and flooding events, stormwater pollution may be impacting some swimming sites," the warning reads.

"Avoid swimming if you can see signs of pollution such as discoloured water and floating debris. Always check with lifeguards for latest beaches conditions.

"Large and powerful surf conditions are expected to be hazardous for coastal activities such as swimming, rock fishing and surfing."

Further north, people should avoid swimming in the Gosford area from Pearl Beach to Forresters Beach.

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A warning has also been issued for some beaches in Port Stephens on the NSW Mid-North Coast, however the water is cleaner in Lake Macquarie and Newcastle.

Down south there is also a warning in place for most beaches from Bulli, through the Illawarra and right down to Gerroa, south of Kiama.

As a rule people are told to avoid swimming the ocean one day after bad storms, but due to the intense weather over the past week this warning may stay in place longer.