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Queenstown Lakes District Council workers yesterday afternoon placed signs around the beach warning people not to enter the water at the Lake Wakatipu bay, after routine water quality monitoring showed "elevated levels of E.coli bacteria".
QLDC chief engineer Ulrich Glasner said the findings could be caused by high winds creating choppy conditions on the lake.
"We are recommending that people avoid contact with lake water and recreational activity such as swimming in the Queenstown Bay area as a precaution for the time being."
The council was resampling water quality yesterday afternoon and test results were expected this morning.
"We will then be able to provide a further update to the community", Mr Glasner said.
"Resampling will take place daily and the bay’s status updated once test results show E.coli levels have subsided."
E.coli can cause diarrhoea, nausea and occasionally vomiting, fatigue and fever.
Holiday-maker Steve Herbing (59), of Melbourne, said it was "a little bit shocking at first" to see the warning signs, considering the usually "pristine waters of this lake".
He said it was unlikely people would want to swim in the lake in the rain but that "people have to be made aware of it".
"I don’t think anybody comes here specifically to swim in the lake. We’ve been here for a few days and it’s quite warm but that water is freezing cold."
He said it was the perception of bacteria levels being high that might concern tourists.
Hundreds of revellers took to the bay to celebrate Christmas Day and New Year’s Eve, and many enjoyed a dip in the lake over the holiday period.
Both residents and tourists were advised to wait until the council gave the all-clear that water quality in that part of the lake was safe before going for a swim.
Otago Regional Council (ORC) water quality sample results completed on December 17 showed Lake Wakatipu at Queenstown Bay had a reading of 816 E.coli per 100ml of water.
The council’s freshwater quality guidelines state that one single sample greater than 550 E.coli per 100ml of water should be considered a possible health risk to swimmers.