The public has been advised not to collect or consume shellfish harvested from the entire Firth of Thames area up to a line from Raukura Point across to Deadmans Point.
New Zealand Food Safety issued a public health warning on the Ministry for Primary Industries website on Wednesday.
Routine tests on shellfish samples taken from this region have shown levels of Paralytic Shellfish Poisoning (PSP) toxins above the safe limit of 0.8mg/kg set by New Zealand Food Safety.
Anyone eating shellfish from this area is potentially at risk of illness.
Mussels, oysters, tuatua, pipi, toheroa, cockles, scallops, catseyes, kina (sea urchins) and all other bivalve shellfish should not be eaten.