The government has given the green light for Transmission Gully to be financed and built by a public-private partnership (PPP).
Transport Minister Gerry Brownlee says the decision means construction can begin in 2014 and the $1 billion, 26km highway between MacKays Crossing and Linden on Wellington's northern corridor will be open by 2020.
"The NZTA (New Zealand Transport Agency) will repay the cost over 25 years, the road can be built now and the costs can be shared by those who will benefit from the project in the future," he said on Wednesday.
Mr Brownlee says tolling won't be part of the PPP contract but the NZTA will "investigate the merits" of using a toll to offset some of the cost.
A decision on tolling will be made separately and the project's start-up doesn't depend on it.
The inland route will cut out Porirua, Pukerua Bay and Paekakariki.
Mr Brownlee says Wellington has been waiting for it for more than 70 years.
"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," he said.
"It will dramatically improve travel times between Wellington and the Lower North Island."