The threat of paralytic shellfish poisoning means eating shellfish from much of Northland's west coast is akin to playing Russian roulette, Northland's Medical Officer of Health says.
Most shellfish species from the Kaipara Harbour north to Maunganui Bluff remain off limits, Jonathan Jarman said.
Kina, mussels, toheroa, pipi, tuatua, oysters, cockles and scallops in the affected area should not be eaten. Paua, crab and crayfish may be eaten if the gut has been completely removed before cooking.
While there have been no confirmed case of toxic shellfish poisoning in Northland for many years, it can potentially be a very serious disease, Dr Jarman said.
At least 20 people have been poisoned after eating toxin-carrying shellfish collected in the Bay of Plenty last week, with 10 hospitalised and four still seriously ill in hospital.
"People have died overseas. Eating shellfish from areas with shellfish warnings is like shellfish Russian roulette,"Dr Jarman said.
"The toxin in paralytic shellfish poisoning is very powerful. It is not affected by cooking and stays just as potent."
The first symptom is numbness in and around the mouth within 20 to 60 minutes of eating affected shellfish.
This numbness spreads to the face and neck and may be followed by difficulty swallowing or breathing, dizziness, double vision and - in severe cases - paralysis and respiratory failure.
The Ministry of Primary Industries (MPI) is also warning people they risk their lives if they eat shellfish from areas where warnings are in place.
Algal blooms occurring at some places on both coasts of the North Island are causing some shellfish to become poisonous, MPI principal adviser Jim Sim said.
"Unfortunately shellfish can be toxic for many months and this is a particular problem for tuatua," Mr Sim said.
"It's impossible to tell from looking at the shellfish which is toxic and which is not, so when warnings go out people need to take note and act accordingly."
New Zealand's commercially harvested shellfish are safe to eat because a marine biotoxin monitoring programme ensures commercial operators don't harvest from areas affected by toxic blooms.
Anyone suffering signs of illness after eating shellfish should seek urgent medical attention, and if possible keep any leftover shellfish for testing.