A public health warning has been issued at a Canterbury river after the discovery of potentially toxic blue-green algae.
The Canterbury District Health Board said the algae was found in the Selwyn/Waikirikiri River, downstream of the Whitecliffs Domain near the picnic area.
Medical officer of health Alistair Humphrey said the algae look like dark brown to black mats, and can produce toxins harmful to people and animals.
He said exposure can cause rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Dr Humphrey said people should stay out of the water until the warnings have been lifted.