People are being warned to avoid parts of the Selwyn River, Waikirikiri, in Canterbury, where potentially toxic algae has been found.
Toxic algae close up under a microscope Photo: Waikato Regional Council
Canterbury District Health Board said people should avoid the Chamberlains Ford area until further notice, and dogs should be kept away too.
Medical officer of health Ramon Pink said the algae looked like dark brown to black mats.
Environment Canterbury is monitoring the sites.