A long stretch of Northland's west coast is now off-limits for shellfish, after tests revealed high levels of the toxin that causes paralytic shellfish poisoning (PSP).
Northland Health is advising people not to collect or eat shellfish from Mitimiti, north of the Hokianga Harbour, to Pouto, at the entrance to Kaipara Harbour.
Symptoms of PSP can include numbness and tingling, difficulty in swallowing or breathing, dizziness and double vision.
Medical Officer of Health Clair Mills says in severe cases, paralysis and respiratory failure could happen within 12 hours of consuming shellfish.
Cooking the shellfish does not remove the toxin.
Anyone who becomes ill after eating shellfish should see a doctor or phone Healthline for advice on 0800 61 11 16.