High levels of blue-green algal bloom has forced the closure of the Waingongoro river mouth in Taranaki.
South Taranaki District Council environmental health officer Geraint Evans said the naturally occurring Cyanobacteria bloom had been causing problems at the river mouth for about a week.
The bloom produces toxins which can cause asthma, paralysis and vomiting or diarrhoea.
Mr Evans said people should not collect or eat fish from the river mouth and avoid contact with the water.
Pets should also be kept away from river mouth, in particular dogs.
Several dogs have died in New Zealand after eating algal blooms.
The river mouth will remain closed until cleared by the Taranaki District Health Board.