It's the height of a giraffe and is going to massively clean up Auckland's beaches and waterways.
At a diameter of 4.5m, the $1.2 billion Central Interceptor is the answer to decades of sewage spewing from old pipes every time it rains and will meet the needs of the city's growing population.
Dubbed the "CI" by Watercare, the giant 13km wastewater tunnel will be built from Western Springs, near Auckland Zoo, to the Māngere wastewater treatment plant.
When the CI and a 2km extension tunnel from Western Springs to Grey Lynn is built, the number of overflows at several central city beaches and waterways will be reduced by more than 80 per cent.
Herne Bay, Sentinel and Home Bay beaches will go from more than 52 overflows a year to 10 or less. It will be the same at a bad overflow at Lyons Ave in St Lukes which sends sewage past a nearby apartment building, along a walkway at the back of Mt Albert Grammar and into Meola Creek.
Water quality will improve at polluted Meola Creek and Pt Chevalier beach, a popular swimming and picnic spot since being resanded in 2009.
The CI will reduce number of overflow points from 219 to just 10 and each of these is expected to overflow less than 10 times a year.
"We want everyone to be able to enjoy clean waterways, beaches and estuaries, that's why we're building the Central Interceptor," says Watercare chief executive Raveen Jaduram.
In older parts of central Auckland, he said, wastewater and stormwater flow into a combined network of pipes. When it rains, stormwater overwhelms the pipes, which are designed to overflow into waterways.
The CI will collect wastewater from several link sewers and shafts along the route, including the Grey Lynn tunnel which has been included within the $1.2b contract at no extra cost.
It is the largest wastewater project in Watercare's history, much bigger than the $118m project to remove the dilapidated sewer that ran across Hobson Bay and replace it with an underground pipeline.
The CI will use a similar German boring machine and technique used at Hobson Bay. The Herrenknecht tunnel-boring machine was selected because of the depth of the tunnel, which will be up to 110m below private properties and parks.
It will run 15m below sea level under the Manukau Harbour.
The existing pipeline that passes under the Manukau Harbour was built in 1964 and is reaching the end of its operational life. Damage to, or failure of the pipeline could result in a major discharge of untreated wastewater into the harbour.
The tunnel-boring machine will arrive in New Zealand in parts and be assembled at the bottom of a shaft at the Māngere wastewater treatment plant towards the end of this year. From there, it will progress at a rate of 15m-20m a day.
It acts like a submarine, ensuring groundwater does not seep into and flood the tunnel.
Earth will be removed via an underground locomotive and rail. Up to 5000cu m of earth a week will be removed as fill for a Watercare project to rehabilitate a former quarry on Puketutu Island on the Manukau Harbour and turn it into a regional park.
The fill will be mixed with biosolids from the treatment plant to replicate the scoria cones that were quarried in the 1950s for projects, including the expansion of Auckland Airport.
A separate, micro-boring machine will work on the link sewers.
When completed in 2025, the CI will be able to store 200,000cu m of wastewater during heavy rainfall, allowing Watercare to control the flow rate to the treatment plant.
The Grey Lynn tunnel is part of a 10-year project to improve water quality in the western isthmus, which includes sewer and stormwater separation in Herne Bay, St Marys Bay, Waterview and pockets of Freemans Bay.
Putting the tunnel into the CI contract is expected to shave $50m to $70m off the $800m western isthmus bill, funded by Watercare and Auckland Council's stormwater division, Healthy Waters.
The western isthmus programme is due to be finished in 2028.