Transmission Gully is being touted as one of the most technically challenging projects in New Zealand's history.
The 27km project, which began in 2015, will run between Kāpiti to south of Porirua when it is completed in April 2020.
Engineers and contractors will shift 8 million cubic metres of dirt, or about four Westpac stadiums full, to flatten the route. In simple terms, they are cutting the tops off the hills to fill in the gullies.
On a recent site visit, Transport Minister Simon Bridges said it was a "beast" of a project which would add resilience to the Wellington region's roading network.
"It's been an incredibly challenging project, simply because of the terrain," he said.
"You can think about other projects that are big: the tunnel work in Auckland at Waterview ... but actually this is probably even more [complex] given the hills - effectively the mountains - that they're going through [and] the sheer quantity of dirt that has to be moved."
The route would add resilience to the Wellington region's roading network, making it more resilient to natural disasters, Mr Bridges said.
Last year, Wellington's major [http://www.radionz.co.nz/national/programmes/checkpoint/audio/201823955/torrential-rain-causes-flooding-slips-in-wellington
transport routes were closed] the day after the November earthquake, when heavy rain brought down unstable hillsides in dozens of places.
The Transport Agency's highway manager for Wellington, Neil Walker, said the new motorway was being built to high seismic standards because of the region's earthquake risk.
"If we had a major seismic event, we could open the road in days rather than months at the moment," he said.
"That's why this project is really important for Wellington. It provides a lifeline into the city."
That, in combination with other roading projects such as the Kāpiti expressway and the Petone-Grenada link road, were "all part of one big system we're building, which is a more resilient network," Mr Walker said.
He said the Transport Agency was identifying other weak spots across Wellington's highway network, such as Ngauranga Gorge, to plan future work.