Health officials in Nelson and Marlborough are warning people not to take or eat shellfish from Port Underwood.
Routine testing has identified the presence of a toxin that can cause diarrhetic shellfish poisoning.
Medical Officer of Health Ed Kiddle says symptoms include diarrhoea, vomiting and abdominal pain. They usually occur within one hour, but can appear up to 12 hours after eating shellfish.
Dr Kiddle says cooking does not remove the toxin, and people are advised not to eat shellfish harvested from affected areas since 22 March.
The warning follows an advisory issued on Monday that Queen Charlotte Sound, including Tory Channel, is closed for shellfish gathering due to paralytic shellfish poisoning.