The Otago Regional Council monitors water quality weekly and says a sample collected yesterday returned a high level of blue-green algae (cyanobacteria) from the upper part of the lagoon.
Signs have been placed at the lagoon today to warn the public of the risks associated with the algae.
Cyanobacteria could produce toxins that were harmful to people and animals if swallowed or through contact with skin.
Exposure may cause symptoms such as skin rashes, nausea, tummy upset, and tingling and numbness around the mouth or tips of fingers.
Anyone experiencing symptoms after contact with contaminated water should visit their doctor.
The Southern DHB's Medical Officer of Health can be contacted on (03) 476-9800 to provide health professionals with information about those symptoms.
Cyanobacteria occur naturally but can increase rapidly during summer months.
If the water is green or has floating bright green or blue scums, all contact should be avoided.
Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the bloom has disappeared.
The Otago Regional Council said it would continue to monitor and update the public as the bloom changes.
