An area of Queenstown's Lake Wakatipu is contaminated with high E.coli levels.
Frankton Bay is one of the 25 sites monitored on a weekly basis for bacteria levels as part of Otago Regional Council (ORC) Contact Recreation monitoring programme.
Southern DHB medical officer of health Dr Keith Reid says swimmers should avoid the area until water sampling tests indicate it's safe.
Exposure to E. coli can result in gastroenteritis symptoms within 12-18 hours of exposure.
"Anyone who swims in the area should wash down with soapy water," he said. "Limiting exposure and practicing good hand hygiene remains the best defence against infection of this kind."
Government water quality guidelines for recreational swimming areas state that those with less than 260 E. coli/100 ml should be safe. Water with more than 550 E. coli/100 ml indicates a potential health risk.
ORC's most recent sampling in the area showed E. coli levels at 921/100 ml.
Further water samples are being collected, with results expected on Monday.