Food safety officials are warning that eating shellfish gathered in Hawke's Bay may lead to paralysis, respiratory failure and even death.
Paralytic shellfish poison produced by some plankton is accumulating in shellfish between the Mohaka River and Cape Kidnappers, 100km to the south.
Routine tests by the Ministry for Primary Industries (MPI) on shellfish taken from the region have shown levels of the toxins above the safe limit.
MPI food safety adviser Piers Harrison said similar warnings had been common in the area for the past three years and could last about three months.
Harrison warned the illness comes on quickly and could lead to paralysis and death.