An unassuming structure in a Whangarei park will be the last major plug in a plan to keep Whangarei Harbour free of sewage.
The 600cum wastewater tank at Tarewa Park follows two similar projects in the city, working towards the goal of having no shellfish bed closures from sewage leaks during major storms.
An existing pipe near the park reserve discharges raw sewage into Waiarohia Stream - which runs into the harbour - during major weather events, usually about three or four times a year.
Whangarei District Council waste and drainage manager Andrew Carvell said the $4.5 million tank - due to be built in 2016 and 2017 - would stop these overflows.
The project budget included increasing the capacity of the sewer line along State Highway 1. By 2017, the council will have spent $30 million over 10 years upgrading its drainage network.
The council had 500km of sewer networks throughout the district, some of which dated back to the 1920s. There was another 500km on private property.
The Tarewa facility is the latest in the council's plan to reduce spilling into the harbour.
The upgrade of the Okara Park pump station and installing a rising main in 2010 also cost $4.5 million. This was followed by the $5 million 1000 cubic litre Hatea Storage Tank and Treatment facility in 2012, which caught any overflow from the Tikipunga and Kamo areas during heavy rain.
While the shellfish beds were once closed about three months a year because of spills, they had not closed since 2012.
Mr Carvell said wastewater discharge had come a long way since 2009, when people took to the streets in protest at the state of the waterways.
There was still work to be done to reduce other sources of river pollution, which was being worked through by the council and the Whangarei Harbour Integrated Management Group.
"The contamination in the summer is all from drystock and birds ... Through sampling you can actually look at the bacteria and find out what animal it's passed through."
The $30 million also covered upgrades to wetlands, and new sewer lines, mostly around Denby Crescent and Kamo Rd. The plant will be buried, covered in grass and largely invisible from the road.