The Otago Regional Council said sampling had shown high concentrations of cyanobacteria (blue-green algae) at the lake, and a health warning had been issued.
Visitors were advised not to use it for recreational purposes, and dog owners should not let their pets swim in the lake until the warning has been lifted.
Lake Waihola has a history of cyanobacteria being present; however, the last time it was present in volumes requiring ORC intervention was three years ago.
Cyanobacteria can produce toxins that are harmful to humans and animals if swallowed or through contact with skin. Exposure to cyanobacteria may cause symptoms such as skin rashes, nausea, tummy upset, and tingling and numbness around the mouth or tips of fingers.
Anyone experiencing health symptoms after contact with contaminated water should visit their doctor and also contact Public Health South on (03) 4769800.
The ORC monitors cyanobacteria weekly at Lake Waihola during summer, and the public will be advised of any changes in water quality that are of public health significance. Signs alerting the public to the presence of cyanobacteria will remain in place until three consecutive samples indicate it is safe.
