People have been advised to stay away from the water at Mollymook Beach after heavy rain caused a sewage overflow.
Shoalhaven Council said diluted sewage was discharged after the sewerage system exceeded its storage capacity amid the heavy rain that hit the region.
There had also been a broken sewerage pipe in the area, which had since been repaired.
Council said water quality was being assessed.
Signs have been erected and water quality results will be available in coming days.
However people should avoid swimming until a further notice.
Council has been working with the NSW EPA and NSW Health in relation to the situation.
Tidal currents, wave action and ocean swells will ensure that the pollution will disperse quickly, however waterways are expected to be impacted for several days.
As a result of the flooding across the Shoalhaven, the community is being advised to avoid swimming if there are signs of pollution such as discoloured water, oil, scum and litter or other debris floating in the water or on the tide line.
In times of heavy rainfall water can also be contaminated with sewage or chemicals, which might continue for some time after the rain subsides.
It is recommended that people do not swim in oceans for 24 to 48 hours after heavy rain, and avoid rivers, lagoons or estuaries affected by floodwaters and runoff for three days.
The NSW State Emergency Service (SES) advises people not to enter floodwater due to hazardous conditions.
For emergency help in floods and storms call the NSW SES on 132 500. In life threatening situations, call Triple Zero (000) immediately.