The $930m, 27km Transmission Gully route north of Wellington has gained final approval today.
The project could be finished by 2021, with a road cut through the hills from Linden to rejoin State Highway 1 north of Paekakariki.
The Environmental Protection Authority this morning released the finding of a board of inquiry meaning the only thing that can stop the road now is an appeal to the High Court on a point of law.
Estimates state it will shave 10 minutes off Kapiti peak-hour commuters' trips to and from Wellington.
It will be one of New Zealand's biggest infrastructure projects and the Wellington region's biggest roading project since the Wellington motorway extension in the late 1970s.
In May, the board of inquiry gave provisional consent for the road but sought feedback ahead of today's final decision.
In its May finding, the board said the existing SH1 route through Paremata and Pukerua Bay was inadequate, congested, severed coastal communities and could close in an earthquake or tsunami.
The new route would ''rectify those inadequacies by providing a new four-lane route which will avoid congestion, reduce travel times and achieve consistency in travel times''.
''The new route will be safer than the coastal route.''
However, the decision also listed environmental fallout, including an increase in sediment to Porirua Harbour, destruction of some indigenous growth, and the destruction, modification, or diversion of 10km of streams.
Sediment from construction and the effects on streams, in particular the nationally significant Pauatahanui Inlet, was of paramount importance, the board said.
Just how much sediment could potentially flush into the inlet was a contentious issue and one that experts could not agree on.
Based on additional information, the board concluded that during construction an additional 3024 tonnes of sediment would enter Porirua Harbour, which is between 4 and 5 percent of the sediment that would flush into it naturally.
There would be about 6 million cubic metres of earthworks, making it the largest earthworks project ever undertaken in the Wellington region.
The draft report said consent should be issued for the main route and Porirua link roads, but it came with more than 100 pages of conditions.
Transpower, one of three applicants for consent, will need to move pylons which hold wires going to Kapiti and Pauatahanui.
It will have to move some power infrastructure but a spokeswoman said there would be no power outages as a result.
Transport Minister Gerry Brownlee has welcomed today's
green-light for the Transmission Gully project, saying it is an
exciting and important milestone not only for the Wellington region
but also for New Zealand's national state highway network.
An alternative state highway route into the capital through Transmission Gully has been talked about for decades, and the Board of Inquiry's final decision to approve the regulatory consent applications will allow the NZ Transport Agency to take the project to the next stage, Brownlee said.
"The Wellington region has been waiting for this day since early last century when the project was first floated, so I'm thrilled to hear a route through Transmission Gully is now set to become a reality."
Wellington is currently reliant on a two-lane highway that has trouble coping in peak times, and is vulnerable to closure in the event of crashes and natural disasters, Brownlee said.
"Our capital city deserves better if it's to reach its full economic potential, and the Transmission Gully route will help to unlock that potential."
United Future leader and Ohariu MP Peter Dunne has also welcomed the final approval, saying years of campaigning for it were now coming to fruition.
"Transmission Gully is absolutely central to securing the future transport and economic needs of the Wellington region," he said.
It is crucial to giving people the kind of access to and from the city that they will need in future.
"It has been a long battle but frankly, this is the only decision that ever made sense and it had to come," Dunne said.
Chair of Greater Wellington Regional Council Fran Wilde says the The Independent Board of Inquiry's decision is great news after almost 50 years of debate about the route.
More than 23,000 vehicles travel daily on SH1 between Kapiti and Wellington City and the road is not coping, Wilde said.
"Levels of congestion at peak hour and at weekends are crippling and the crash rates on some parts of this route are unacceptable.
"In recent years we have had more frequent examples of the unreliability of the Centennial Highway, not just closure because of crashes but also extreme weather events.
The new Transmission Gully route, which has higher seismic resilience than the present route, will help to "future proof" the region, Wilde said.