Northland Health advises people not to gather or eat shellfish from parts of the Bay of Islands because of the risk from sewage pollution.
The warning extends from Opua to Te Haumi and Orongo Bay and will be in force for at least a month.
Health protection officer Tahi Morton said there was a risk of faecal contamination after recent sewage overflows into the harbour areas from the Far North District Council's Kawakawa and Russell treatment plants.
"We have erected warning signs near the potentially contaminated areas advising of the risk to public health arising from the spills."
Mr Morton said people who ate contaminated shellfish were at high risk of contracting illnesses that caused diarrhoea and vomiting, including norovirus, campylobacter and salmonella.
"Young children and older people in particular can become seriously ill as the result of these infections."
Anyone experiencing these symptoms after eating shellfish is advised to see a doctor immediately.
The warning comes after Northland Health ordered an immediate closure of several oyster farms in the area after between 6cu m and 30cu m of raw sewage leaked into a stream leading to Uriti Bay.
The leak came from a broken pipe joint on a pressure main to the council's wastewater treatment plant at Russell, and happened just one week after 23,000cu m of partly treated sewage was discharged into the Kawakawa River, which flows into the Bay of Islands.
The council has asked Northland Health to carry out water tests at Orongo Bay so the farms can re-open as soon aspossible.