Nelson Marlborough District Health Board is telling the public not to gather shellfish from Port Underwood, west of Robertson's Point, in Marlborough, to avoid eating poisonous toxins.
The DHB says routine tests found high levels of an algal bloom that causes diarrhetic shellfish poisoning.
Symptoms include diarrhoea, nausea, vomiting and abdominal pain.
The board cooking does not remove the toxin and anyone who has symptoms after eating the shellfish should obtain medical help.