Freshwater Beach was one of many closed to swimmers on Saturday, with people urged not to enter the water because of excess pollution from a night of heavy rain.
Curl Curl, Bronte, and Coogee beaches were also impacted.
The deluge of rain overnight also left most bathing spots along Sydney Harbour with a bad pollution ratings, leading to advice against swimming there also.
When there is heavy rainfall, particularly during a storm, oceans can fill up with stormwater pollution.
As long as the stormwater remains in the water, the NSW Office of Environment and Heritage, recommends that oceans and beaches should be avoided until the pollution clears.
The most obvious indications of stormwater pollution include water discolouration and debris in the water and on the tide line.