A gastro bug contamination and a spate of illnesses have prompted the Ministry for Primary Industries to issue a warning about eating chilled oysters collected from a harbour north of Auckland.

Four seafood companies are recalling all oysters harvested from the Mahurangi Harbour - near Warkworth - from mid January, with authorities saying the area has likely been contaminated with the bug norovirus.

The companies are Matakana Oysters, Orata Marine Limited, Seafood Harbour, and Hutchings and Addison Limited.

Local health authorities have reported 11 people becoming ill from eating oysters in the area, MPI's food compliance service group manager Melinda Sando said.

"It is likely to be the result of a localised contamination - for example discharge from a boat or a leaking septic tank," she said.

"Samples of the oysters were tested by ESR and norovirus was found. The type of virus is the same as that found in the affected people."

The ministry is calling for anyone with frozen oysters from the area in their freezer to throw them out.

The oysters are not exported.

All harvesting in the area has been halted while the contamination is investigated.

Ms Sando said it appeared the contamination was limited to a small harbour area around Dyers Creek.

The symptoms of norovirus gastroenteritis include vomiting and diarrhoea.