The Queenstown Lakes District Council says people should avoid swimming and recreation at Lake Hayes after water testing at the lake's northern end showed levels of E.coli bacteria more than four times the acceptable level.
Government water quality guidelines for recreational swimming areas state water with more than 550 E.coli per 100ml indicate a potential health risk.
The most recent sampling at Mill Creek Shallows showed levels at 2420 E.coli per 100ml.
Warning signs were put up in the area yesterday.
"We're recommending people avoid swimming and recreational activity at Lake Hayes for the time being, as a precaution," Queenstown Lakes District Council chief engineer Ulrich Glasner said.
More tests have been carried out and the results are expected today.
