Auckland’s biggest ever wastewater project is now carrying flows along its entire length, seven years after the first diggers arrived. Watercare confirmed on Monday that the northern section of the Central Interceptor has gone into service, making the full 16.2 kilometre tunnel operational and completing a $1.66 billion build that began in 2019.
The tunnel runs from Point Erin in Herne Bay to the Māngere Wastewater Treatment Plant, sitting between 15 and 110 metres below ground and passing beneath the Manukau Harbour on the way. It is 4.5 metres across, wide enough to drive a truck through, and gravity does the work. Wastewater and stormwater that would once have spilled into streams and onto beaches during heavy rain now runs downhill through the tunnel to Māngere for treatment.
Watercare chief executive Jamie Sinclair said the completion was a turning point for the city. “The Central Interceptor is a game changer for our city that will deliver lasting environmental, community and growth benefits for Aucklanders,” he said in a statement released by Watercare.
“This project is at the heart of our commitment to improving water quality across the region. Bringing together world-class engineering, long-term planning and strong partnerships, it delivers benefits for Aucklanders now while enabling a new generation of projects that will further reduce wastewater overflows and make a real difference to the health of the Waitematā Harbour and other waterways into the future.”
The southern half of the tunnel has been running since early 2025 and the numbers from that period give some sense of the scale involved. Watercare estimates it stopped 741,000 cubic metres of wastewater and stormwater going into the environment by 1 July this year, which the company puts at close to 300 Olympic sized swimming pools. That was with only part of the system switched on.
The problem the tunnel is built to fix is an old one. Large parts of central Auckland still use combined pipes that carry sewage and stormwater together, a design laid down when the city was much smaller. In dry weather the arrangement works. In heavy rain the pipes fill, and the overflow has to go somewhere, which historically meant it went into the nearest stream or straight out to the harbour. Anyone who has checked Safeswim after a downpour and found a black pin next to their local beach has seen the result.
Auckland Council head of network planning Nick Vigar set out the numbers in June, when the tunnel’s two halves were joined. “Currently, there are hundreds of overflow points into the Waitematā Harbour along the inner harbour’s northern coastline. Some beaches can experience many dozens of wastewater overflow events per year, which are shown on Safeswim as black pins,” he said in an Auckland Council update published on 5 June. “When the Central Interceptor and supporting works including the Herne Bay Collector are fully operational, it is predicted that there will be around six.”
That last condition matters. The tunnel on its own does not finish the job. The Herne Bay Collector, a 1.7 kilometre tunnel that will feed the oldest combined pipe networks in the inner suburbs into the Central Interceptor, is still being built. Watercare signed Italian tunnelling firm Ghella to deliver it earlier this year. Until that connection is made, some of the predicted improvement for inner harbour beaches remains ahead rather than banked.
Mayor Wayne Brown, an engineer by background, described the project as a significant achievement for the city. “This is Auckland’s largest wastewater infrastructure project, taking seven years to complete using a tunnel boring machine to build the gravity driven tunnel across the isthmus between Māngere in the south and the central city,” he said.
“Aucklanders want cleaner waterways and to be able to enjoy the likes of Browny’s pool, local beaches and marine environment. Delivering these outcomes while creating a smarter, more resilient network is exactly what we are committed to providing for the future of Auckland. This project shows what’s possible when we invest in the right infrastructure at the right time.”
The delivery record is the part infrastructure watchers are likely to dwell on. Watercare says the project came in on schedule and close to its original budget, despite running through the Covid-19 pandemic, several major weather events and a stretch of unusually high construction inflation. Sinclair acknowledged how unusual that is. “Mega projects of this scale rarely stay so close to programme and budget, particularly given the challenges faced throughout delivery,” he said.
Set against the cost overruns that have dogged other large New Zealand builds, a $1.66 billion tunnel finishing roughly where it said it would is worth noting. The work was delivered by Watercare with the Ghella Abergeldie Joint Venture, alongside mana whenua, Auckland Council and the communities living above 17 construction sites and 19 shafts scattered across the isthmus. More than 4,300 people worked on it.
The machine that dug it became something of a local celebrity. Hiwa-i-te-Rangi was 190 metres long, weighed 740 tonnes and was named by school students after the youngest star in the Matariki cluster, associated with growth and prosperity. She ran on electricity rather than diesel, as did the locomotives that hauled spoil out of the tunnel, cutting fuel use over the years of digging.
The finished tunnel can hold 299,000 cubic metres of wastewater and stormwater at once, which is what allows it to absorb a storm surge rather than passing the problem downstream. It also links to the network through smaller sewers in Mount Roskill, Blockhouse Bay and Mount Albert.
Watercare marked the milestone with an event on Monday, with a karakia whakamutunga, a final blessing to close the project, scheduled for Tuesday morning. We reported in June on the removal of the final bulkhead, the construction step that joined the two halves into a single continuous tunnel and set up this week’s switch on.
The real test comes with the next big rain. Aucklanders will find out over the coming winters whether the black pins on Safeswim start disappearing.
Have you noticed a difference at your local beach since the southern half of the tunnel went live? Tell us in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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