Incredible flyover footage: The new $1.25b Transmission Gully highway in and out of Wellington

Construction of Wellington's troubled Transmission Gully motorway is now more than 85 per cent complete and NZTA is confident it will open next September.

The 27km road has suffered delays, cost blowouts, and is now the subject of a Government-ordered review.

It was meant to be open by April 2020, then it was pushed back to before Christmas this year, and now it's due to open in September 2021.

The project is being built through a public-private partnership, the Wellington Gateway Partnership (WGP), with CPB Contractors and HEB Construction sub-contracted to carry out the design and construction.

The total cost of the road has climbed to $1.25 billion after NZTA bailed out contractors twice due to issues including the Kaikōura earthquake, extreme weather events, and the Covid-19 pandemic.

But Wellington Gateway Partnership chief executive Sergio Mejia said today he could see the light at the end of the tunnel and was confident the road would open next September.

"I'm certain we're going to hit the target, we're fully committed and we're on schedule."

Wellington Gateway Partnership chief executive Sergio Mejia and Waka Kotahi NZTA project delivery senior manager Andy Thackwray. Photo / Mark Mitchell

Mejia said he was proud of the team working on the four-lane motorway that will run from Mackays Crossing to Linden.

"Regardless of how difficult things are and have been, the guys keep going and keep working.

There will be some activity on the site over Christmas, but crews will take a break before coming back to go "full steam ahead", Mejia said.

"You'll see the crews laying pavement and the road will be largely completed mid next year."

Waka Kotahi NZTA project delivery senior manager Andy Thackwray acknowledged Wellingtonians have waited a long time for the road to open.

When asked how the transport agency would rebuild public confidence in the project, Thackwray said: "It's a very difficult and complex project to deliver, we are on track to open next year and we'll stay on track."

Running from north to south, the first part of the alignment is where there is the most work left to do.

The largest excavation cuts have been made at this end of the road, where the Wainui Saddle is.

The Transmission Gully construction site at Wainui Saddle. Photo / Mark Mitchell

The cuts here are up to 70m deep resulting in the removal of more than 865,000 cubic metres of earth.

That amount equates to almost three times the volume of Wellington's Sky Stadium.

During the Kaikōura Earthquake a slip came down in this area, which proved to be just one of many setbacks for the Transmission Gully build.

The face of the earth, rising high around where the road will eventually be, is smothered in 22,700sq m of specialised concrete.

Piles of pipes lie on the side of the road corridor, which is bumpy and tricky to drive over.

It's clear this road was nowhere close to being open by Christmas.

Further along from the saddle the hills start to disappear and the site gets flatter, but that by no means makes the build any easier.

The earth here is boggy and wet so a lot of it had to be removed and replaced with more stable material.

When Transmission Gully opens, about 25,000 vehicles are expected to use it each day. Photo / Mark Mitchell

Mountains of gravel have been stockpiled on the side of the road.

The gravel will eventually be mixed with cement and layered on to the road. About 780,000 tonnes of crushed rock is needed for pavement construction at Transmission Gully.

It's truck city further along at the Waitangirua Interchange.

At the moment there are 57 tipper trucks working on site to move the pavement material around to where it's needed.

Farmland surrounds the alignment and little access roads have been built underneath the main Transmission Gully route to maintain accessibility.

Up to 7km of local streams have been diverted and ecologists have trapped and transferred 31,000 fish to ensure they didn't get disturbed by construction work.

It's hoped that in less than 10 years after the new motorway opens, the water quality and surrounding environment will be in a better state than before the motorway was built.

The Transmission Gully construction site at the Cannons Creek bridge, north of Wellington. Photo / Mark Mitchell

Further down the road lies the largest single structure in the project, which is the bridge over Cannons Creek, called Te Ara a Toa.

The bridge is four lanes and stands 60 metres above the stream below.

It took six years to construct the 230m-long bridge, which is base isolated to reduce movement when the surrounding earth shifts in an earthquake.

More than 9 million work hours have been completed on the project to date and there are currently 570 people working onsite.

When Transmission Gully opens, about 25,000 vehicles are expected to use it each day.