The contract to build a billion-dollar tunnel beneath Auckland will be signed on Thursday.
The Central Interceptor will run for 13km beneath the city from Western Springs to Mangere, reducing overflows in wet weather by an estimated 80 percent.
It'll be located between 22 and 110m below the surface, connecting to the existing wastewater treatment system. Its 4.5m width will give it plenty of capacity to handle excess flows.
Council-controlled organisation Watercare and a joint venture between construction firms Ghella and Abergeldie Harker will sign the $1.2 billion deal on Thursday at 11am.
"Parts of the old Auckland City Council area have no stormwater system, so when it rains the stormwater goes into the wastewater pipes and then overflows into streams and beaches," Watercare chief executive Raveen Jaduram said in November, when the deal was announced.
"This project will improve the health of our city's waterways by reducing wet-weather overflows and provide for population growth. Not only that, its construction will create employment and opportunities for the city."
The Ghella-Abergeldie Harker joint venture got the nod over four other bids.
In preventing overflows, the Central Interceptor will improve water quality at some of the city's beaches.
It's the largest wastewater project in Auckland's history, and will be completed in 2025.
Water bills across the city are expected to rise about 3 percent a year to help pay for the Central Interceptor and other upgrades to the city's networks, NZME reported.