Health authorities in Marlborough say a toxin remains at dangerously high levels in shellfish in Tory Channel.
A health warning issued before Christmas will stay in place after monitoring showed the toxin known as paralytic shellfish poisoning, or PSP, is still present.
Nelson Marlborough medical officer of health Ed Kiddle said the toxin could cause serious illness.
"For some people it might just cause tingling around their mouth, or on their face or their arms.
"But it can move on an cause some more serious symptoms such as difficulty in breathing and getting dizzy, and even paralysis and respiratory failure."
Mr Kiddle said people should not eat any kina, mussels, pipi, tuatua, oysters or cockles harvested from Tory Channel.
He said paua, crab and crayfish can still be eaten, but only if the gut is completely removed before cooking.