The contract to build a new $27 million water treatment plant at Whangārei's Whau Valley which will secure the city's urban water supply for decades has been awarded.

The plant will be at 270 Whau Valley Rd and supplied with raw water from the existing Whau Valley Dam and the Hatea River.

The contract worth $26,977,568.29 plus GST was awarded to Broadspectrum at a full council meeting last week.

Infrastructure general manager Simon Weston said the new plant would increase the water supply from 15,000 cubic metres processed each day by the existing plant to 22,000 cubic metres a day and would involve new filtration systems and the latest technology.

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"It will be modern, future-proofed, more robust and provide greater resilience when it comes to water security. This is especially important considering the growth we are experiencing, and far greater challenges for those involved in the water supply business over the next few decades," Weston said.

He said it has been a "long and involved" process to bring the council to the point where the contract can be awarded and thanked all those involved.

The council has also agreed to sell a piece the land at 213 Whau Valley Rd which it purchased in case it was needed as a site for this project. The proceeds from the sale, which must be no less than market value, will be transferred to the new plant project.

Instead the council has opted for the "better" site at 270 Whau Valley Rd. All necessary consents have been granted by Northland Regional Council.

The existing plant, which has been providing water to Whangārei for more than 60 years, will continue to operate until the new plant is up and running. After that a decision will be made about its future decommissioning.

"Earthworks will begin on the new site late this month and the build, including treatment processes and amenities building and extensions of pipeline along Whau Valley Rd to connect to the existing reservoir, should be completed by the end of 2020."