The pressure is mounting, both mentally and physically, as workers dig towards Auckland's Manukau Harbour in effort to build a tunnel from Māngere to Grey Lynn.

The Central Interceptor wastewater pipe project is the largest tunnel ever bored in New Zealand and will allow an additional 360,000 houses to be built in Auckland.

Graduate engineer Mark Boules said engineers would be working 24 hours a day to get through the harbour stretch of the tunnel quickly.

“From an engineering point of view, it is challenging to mine beneath a giant body of water. It’s nerve-wracking for sure, but these people are the best in the world at what they do,” he said.

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Engineers have to carefully balance the internal pressure of their giant tunnel boring drill with the pressure created by the soil above, otherwise water could come pouring in through fissures.

The machine can never be turned off and so a tunneller’s shift is 10 hours long underground, with a day and night change-over cycle.

It can reach a temperature of 30C in the tunnel because of heat generated by the machinery and by the Earth.

“I do miss the sunshine, but, in winter when it's storming outside, it’s comforting to go into the tunnel because no weather affects you down there,” Boules said.

The job is Boules’ first after having graduated with a civil engineering degree.

He said it was much more exciting than his internship pushing paper at Auckland Council.

Each day the team removes around 900 tonnes of soil, producing about 16 metres of tunnel.

The soil is transported out to the surface on “muck wagons” towed by a small train.

Kiwi engineers working on the interceptor have impressed their Italian joint venture partners with their ingenuity, having hung up Christmas lights in the tunnel which show the train driver where to stop.

On the return trip, the train tows in concrete segments, which form the tunnel. The concrete is plated with steel to protect it from being dissolved by the human faeces it will eventually carry.

The tunnel’s internal diameter is 4.5 metres and space is tight when working alongside the rail tracks or the 190 metre-long tunnel boring machine.

“You certainly get to know people well down there, because you are in close quarters,” Boules said.

The tunnelling machine has several gantries which serve various purposes. One of them has a break room where tunnellers eat their lunch, sitting in single file.

Boules said it had a bathroom reserved for “emergencies”, which was “like a Portaloo, but smaller”. He described it as being like a chamber pot that tunnellers squat on.

Tunnellers are prepared for emergencies of all kinds.

Anyone who enters the tunnel takes in a “self rescue box” which converts carbon dioxide and the moisture from breath back into oxygen through a chemical process, providing breathable air through a tube.

It can provide half an hour of oxygen which ought to be enough to reach one of three pressurised “refuge chambers” which have 36 hours of oxygen and snacks.

Central Interceptor executive programme director Shayne Cunis said that, all up, the project involved creating 20 kilometres of tunnel, as compared to the City Rail Link’s 3.45 kilometre tunnels.

The interceptor tunnel will run beneath 1500 houses, although, at 35 metres underground, residents will not feel shakes, Cunis said.

Cunis said that New Zealand needed to be already planning for it’s next big project in order to get efficiency from the tunnelling industry.

“There was a huge gap between this and the Waterview tunnel. We need to begin the next project as soon as this one finishes in 2026, to utilise equipment and keep expertise on shore.”

He said projects on the horizon that could require tunnelling were light rail to the airport, a harbour crossing or even a tunnel underneath the Kaimai ranges for easier access to Port of Tauranga.

“Tunnelling is often the answer to solving projects that have been put into the ‘too hard’ category.”

CORRECTION: An earlier version of this story said the tunnel was under Manukau Harbour, but work has not reached the harbour yet. Digging happens 24/7 because the boring machine cannot easily be powered down, not because there is any danger of tunnel collapse. (Amended at 9.03am on June 3, 2022)