A potentially toxic blue-green algae has been found in the Selwyn River and Lake Ellesmere in Canterbury.
Local District Health Board medical officer of health Dr Alistair Humphrey says people should not drink water from the river at the Whitecliffs Domain or from the lake.
He says people should also stay out of the water as contact with the algae may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Boiling water does not remove the toxins and the algae is particularly dangerous for dogs.
The sites are being monitored by Canterbury Regional Council.