A tunnel boring machine, named Amiria, has been digging out the last leg of a $400 million Watercare project which will ensure water comes out when Aucklanders turn on a tap.
The tunnel stretches 31km from a set of Watercare reservoirs in Manukau to those atop Khyber Pass.
About midday Thursday, Amiria popped out "bang on" where she was meant to at the top of the Khyber Pass reservoir.
Inside the tunnel will live an earthquake-proof water main with the capability of quenching 25 per cent of the city's thirst everyday.
Its technical capacity is 120 megalitres a day – the equivalent of filling 48 Olympic swimming pools.
Instead of following the southern motorway like other water mains, the tunnel – "Hunua 4" – heads west to Auckland Airport before zipping under the Māngere Bridge and snaking around One Tree Hill.
Project manager David Moore said that would add extra resilience to Auckland's drinking water supply in case of natural disaster.
"If we had an earthquake, we've built it in a very different route to previous ones. We've crossed the harbour, which has been a weak spot."
Construction on the tunnel began in 2008, and was broken down into 11 phases.
About 27km were trenched straight into the ground, 1km was suspended under the Mangere Bridge, and the last 3km, between Newmarket and Khyber Pass, were to be bored.
Despite being considerably more expensive and time-consuming than trenching, burrowing underground using a tunnel boring machine negates the need to rip up the road and cause traffic chaos.
That was a necessary evil given that Khyber Pass Rd sees an average of 30,000 vehicles travelling its length every day.
Now the Khyber Pass tunnel section is complete, there are just two further mini tunnel sections to be built: a stretch in Epsom and another in Newmarket.
Moore said once everything was complete, the main would ensure that Watercare could continue to provide uninterrupted, top quality water as Auckland grew.
"We take for granted that we can turn on a tap and water comes out, what we don't think about is the huge amount of infrastructure, labour, planning and technology that goes on in the background to ensure that happens.
"It's really really key for us to have this new water main ... it will help the North Shore, West Auckland, the whole of the city and the south as it comes through."
Watercare supplies water to an area stretching from the Bombay hills in the south, Wellsford in the North and Waiuku in the west.
The council-controlled organisation collects water from places like the Waitākere and Hunua Ranges in the form of dams, or from the Waikato river, and treats it before piping it to individual users.