Health authorities in Hawke's Bay are warning people to avoid any contact with water in the Ahuriri estuary.
They say test results show very high levels of E-coli contamination after heavy rainfall and wastewater discharge into the estuary earlier this week.
Medical officer of health Nick Jones said further tests would be done, but in the meantime people should not swim, collect shellfish or undertake any other water-based recreational activities in the area.