For the second time this summer people are being warned not to collect or eat shellfish from Northland's west coast between Mitimiti just north of the Hokianga Harbour mouth to Pouto at the entrance to Kaipara Harbour.

Shellfish collected within the area have shown unsafe levels of paralytic shellfish poisoning toxins.

"Cooking the shellfish does not remove the toxin," said Dr Virginia McLaughlin, Northland Medical Officer of Health.

A ban from the same area of coast was lifted on New Year's Eve after being imposed in early December. A top marine biology chemist warned then that people who risked eating toxic shellfish from Northland's west coast could "end up with more than just the poos and spews". Tim Harwood, from the Cawthron Institute in Nelson, said people around the world die every year from paralytic shellfish poisoning, although serious cases were rare in New Zealand.

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Symptoms include numbness and tingling around the mouth, face or extremities; difficulty in swallowing or breathing; dizziness; and double vision.

In severe cases, paralysis and respiratory failure can occur within 12 hours.

- Anyone who becomes ill should see a doctor, or phone Healthline on 0800 611 116 for advice or, in emergencies, phone 111.