The entire Manukau Harbour is now off limits for shellfish gathering after a spike in toxin levels.
Paralytic shellfish poisoning toxins have risen "significantly" in the area, and eating the shellfish is now considered dangerous.
The toxins can cause symptoms ranging from tingling, nausea, dizziness through to paralysis, respiratory failure and even death, with symptoms usually appearing within 10 minutes to three hours after eating.
Cooking does not get rid of the toxin.
Mussels, oysters, tuatua, pipi, toheroa, cockles, scallops, catseyes, kina (sea urchin) and all other bivalve shellfish are all unsafe to eat, MPI said.
But pāua, crab and crayfish can still be eaten if their gut has been completely removed prior to cooking, as toxins accumulate in the gut. If the gut is not removed its contents could contaminate the meat.
In April a ban on shellfish gathering was issued for a swathe of the Waikato, and at the beginning of October MPI issued a public health warning not to gather shellfish from Awakino down to Oakura in Taranaki.
On November 1 MPI extended the warning through to the Manukau Harbour's South Head, but said shellfish in the harbour were still safe to eat.
But the public health warning has now been extended to include the entire Manukau Harbour through to Huia.
Shellfish samples are tested weekly for toxin levels, with the most recent tests showing the toxin is above the safe limit of 0.8mg/kg.
MPI said it would continue testing and let the public know if the shellfish become safe to eat.
Anyone who becomes ill after eating shellfish from an area where a public health warning has been issued should phone Healthline for advice on 0800 61 11 16, or seek medical attention immediately.
They should also contact their nearest public health unit and keep any leftover shellfish for testing.