The public is being warned not to take or eat shellfish from Queen Charlotte Sound, in the Marlborough Sounds.
Routine testing by the Nelson-Marlborough District Health Board has shown higher than acceptable levels of toxins that can cause poisoning.
Medical Officer of Health Doctor Ed Kiddle says the affected area covers all of Queen Charlotte Sound, including the Tory Channel.
He says cooking the shellfish does not remove the toxin, and symptoms of poisoning include numbness, dizziness or difficulty swallowing.
Fish from the area are not affected by the toxin and are still safe to eat.