Auckland's Central Interceptor Project has passed a major milestone, with Watercare's giant tunnel boring machine reaching the end of its 16.2 kilometre journey underneath the city.

The Tunnel Boring Machine, named Hiwa-i-te-Rangi, broke through a shaft in Herne Bay this morning.

It began its journey in July 2021 at the Māngere Wastewater Treatment Plant.

Auckland Mayor Wayne Brown gave the order for Hiwa-i-te-Rangi to start up for the last time to finish the final few centimetres, and its breakthrough was projected onto a large screen for onlookers.

It was met by loud cheers.

Watercare chief programme delivery officer Shayne Cunis said the breakthrough was was testament to the skills of the tunnelling team.

He said 600 staff have worked on the project.

"The tunnellers have worked 24 hours per day, six days a week to build the tunnel. This is world class engineering, and it's been completed safely, economically and on time, which is rare for any major infrastructure project," he said.

In coming weeks, Watercare says Hiwa-i-te-Rangi will be removed from the shaft in sections, and crews will undertake thermal welding on tunnel segments to make sure the the tunnel is air-tight.

The $1.6 billion dollar Central Interceptor is New Zealand's longest wastewater tunnel, and is expected to reduce wet weather overflows by 80 percent.

It's due to be finished next year.

This story was originally published by RNZ