Canterbury people are being warned to stay away from the Rakahuri/Ashley River due to dangerous toxic algae in the water.
People and animals should not go towards the river while a health warning is in place, according to the community and public health division of the Canterbury District Health Board.
The DHB issued the warning after discovery of detaching mats of potentially toxic blue-green algae (benthic cyanobacteria) in the river.
Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
The algae look like dark brown to black mats and can produce toxins harmful to people and animals, Canterbury Medical Officer of Health Dr Ramon Pink said.