A popular swimming spot near Temuka in South Canterbury has been closed due to high levels of a toxic algae.
Just a teaspoon of cyanobacteria can kill a dog and so far 100 have died after swimming in waterways affected by it.
The algae was now covering the bed of the Opihi River at state highway one in thick black mats.
Signage was due to go up and health authorities were warning people not to swim there or let their dogs in the water.
Symptoms included skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Anybody feeling ill was being asked to contact a doctor immediately.
Those seeing sick dogs next to rivers were asked to take them to their nearest veterinarian.
About 2000 Temuka residentsrecently discovered their drinking water was contaminated with asbestos from faulty pipes.
The council said their water was still safe to drink.