A ban on taking or eating shellfish from Queen Charlotte Sound is in place after high levels of toxins were discovered.
Routine testing has shown increased numbers of the phytoplankton that can cause paralytic shellfish poisoning.
Nelson-Marlborough medical officer of health Jill Sherwood says symptoms of poisoning include numbness and tingling around the mouth, face or extremities, difficulty swallowing or breathing, dizziness and double vision.
Dr Sherwood says in severe cases, paralysis and respiratory failure can occur and acute symptoms usually occur within 12 hours of eating shellfish.
People are being advised not to eat kina, mussels, pipi, tuatua, oysters and cockles harvested from the affected areas since 19 February, even if they have been frozen or cooked.