Health authorities are warning people to steer clear of Waipu estuary in Northland, where red algae blooms have killed hundreds of fish.
Waipu Estuary. Photo: 123RF
Northland Regional Council says the dying blooms reduced dissolved oxygen in the water and suffocated the fish.
It says a large section of the estuary - 42 kilometres south of Whangarei - is affected.
As the fish decompose, they are, in turn, releasing hydrogen sulphide gas, with a "rotten egg" odour.