Health agencies in Canterbury are warning of potentially toxic algae in the Ashley River.
Canterbury District Health Board says people should avoid the popular swimming river upstream of State Highway One until the warning has been lifted, and dogs in particular should be kept away.
Medical Officer of Health Ramon Pink said the algae look like dark brown or black mats and can produce toxins harmful to people and animals.
He said exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Canterbury Regional Council is monitoring the sites.