The Ministry for Primary Industries is warning of a gastro bug risk from rock oysters harvested in a harbour just north of Auckland.
It was alerted to the issue by local health authorities after 11 people became ill after eating oysters from the Mahurangi Harbour near Warkworth.
Matakana Oysters Limited, Orata Marine Limited and Seafood Harbour and Hutchings and Addison Limited are recalling all oysters harvested since mid-January.
MPI's Manager of Food Compliance Service Group, Melinda Sando, said samples of the oysters were tested and norovirus was found.
She said it was likely to be the result of a localised contamination - such as discharge from a boat or a leaking septic tank.
MPI said anyone with recently harvested oysters in their freezer should throw them out.
"It is quite a small amount of oysters that are potentially still out there. We think that most of them would have been consumed by now," Ms Sando said.
"Our main concern, however, is that people may have frozen the oysters and still have them in their freezer. It is very important that they are discarded."
Ms Sando said the problem appeared to be limited to a small area of the harbour around Dyers Creek.
MPI has put a halt on all harvesting of oysters from the harbour as it investigates.
Norovirus gastroenteritis generally presents with vomiting followed by diarrhoea. The Ministry advises people who believe they have eaten affected oysters to see their GP if symptoms are severe or persistent or if they are unable to keep hydrated.
Information was available through the Ministry of Health's Helpline 0800 611 116.
