People are being warned to avoid contact with a popular Queenstown lake after high levels of E coli were discovered.
Water quality samples from Lake Hayes Mill Creek Shallows showed levels at 2420 E coli per 100 millilitres.
The government's guidelines for recreational swimming say water with more than 550 E coli per 100ml is a potential health risk.
Queenstown Lakes District Council chief engineer Ulrich Glasner said signage would be in place later today at multiple locations to warn people to avoid contact with the water.
"We're recommending people avoid swimming and recreational activity at Lake Hayes for the time being, as a precaution," Mr Glasner said.
Symptoms of E coli exposure include severe stomach cramping, fatigue, vomiting and diarrhoea within 18 hours of contact.
The source of the increased levels is unknown.
"We have already started the resampling and hope to have the test results back tomorrow. We will then be able to provide a further update to the community," Mr Glasner said.
"Resampling will take place daily and the status updated once test results show E coli levels have subsided."
Anyone concerned about their health or someone else's health should contact a doctor or ring HealthLine on 0800 611 116.