Swimmers and other users of Wellington Harbour and waters off Wellington's South Coast are being warned to stay out of the water for another 24 hours following sewage contamination caused by yesterday's heavy rain.

Pollution-warning signs had been erected in Lyall Bay and around Moa Point after sewage, heavily diluted by stormwater, was discharged down the long outfall from the Moa Point treatment plant at the height of the storm, Wellington City Council said today.

Regional Public Health Medical Officer of Health Dr Stephen Palmer said caution was the best approach and recommended that people not swim in coastal recreational waters near urban areas for 48 hours after heavy rainfall because of possible health effects from stormwater and sewage overflow contamination.

The council, in conjunction with Regional Public Health, has done water-quality sampling and will continue to do so over the next few days.