The government's decision to allow Transmission Gully to be financed and built by a public-private partnership could put the $1 billion highway project at risk, opposition parties say.
Transport Minister Gerry Brownlee says it means construction can begin in 2014 and the 26km bypass 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.
The inland route will cut out Porirua, Pukerua Bay and Paekakariki.
Labour's transport spokesman, Phil Twyford, says the government's timing couldn't be worse.
"Four big Australian public-private partnerships (PPPs) have collapsed or are in the process of falling over," he said.
"It's hard to imagine there would be any private sector investors willing to take on the patronage risk in such a project... they are fraught with problems."
The Greens say a PPP is an expensive form of borrowing and the highway will be an uneconomic burden.
"The government is going to have to repay the consortium over 25 years, plus interest and profits," said transport spokeswoman Julie Anne Genter.
"Time and again costs blow out, traffic forecasts are not met, private consortia collapse and it is the taxpayer who is left footing the bill."