Shellfish are off the menu and swimming is banned in Whangarei Harbour after a failure at the Whangarei Wastewater Treatment plant pumped up to 930 cubic metres of sewage into the harbour.
Due to heavy rain over the past three days the Northland Medical Officer of Health has advised people not to swim in Whangarei Harbour for five days and not to gather shellfish in the harbour - including Mair and Snake Banks - for the next 28 days following the spill from the treatment plant.
Whangarei District Council said the spill would have been far worse if it hadn't been for recent upgrades to the city's sewerage system.
"We've made huge progress over the past four years so there was no uncontrolled spill this time, but our main sewage treatment plant did go into extreme bypass mode for two hours," council waste and drainage manager Andrew Carvell said.
"By this time next year we will have expanded the plant enough for extreme bypass to become a thing of the past. Under extreme bypass some wastewater going into the plant for treatment gets minimum processing before being discharged.
"In this case around 2 per cent of the flow into the plant was only screened and settled before being discharged, and did not get disinfected."
"On the other hand, this storm has shown the major progress the council is are making across its sewerage system," Mr Carvell said.
An extension to the disinfection process completed last month reduced the extreme bypass volume from 6400 to 930m3, he said.
"The construction of an additional disinfection process to treat all wastewater is currently being designed, with a completion date planned for May next year," Mr Carvell said.
"During the storm the new $5 million Hatea wastewater storage and treatment plant passed its first real test with flying colours. The new tank stored 300 cubic metres of untreated wastewater that would have spilt directly into the Hatea River previously. This has since been pumped back into the wastewater system and treated at the Kioreroa Rd wastewater treatment plant.
"The new plant that includes a large storage tank, ultraviolet light disinfection and a new pump, is now able to deal with a million litres of wastewater."
Mr Carvell said no other spills occurred during the storm, although there was widespread pressure on the system. He said 120mm of rain fell in Hatea River catchment during the weekend.
The harbour has been now closed because of sewage contamination three times since last October.