The proposed Warkworth to Wellsford motorway north of Auckland could cost more than double the $1 billion figure of two years ago, latest estimates show.
The 27km motorway - still in its planning stages - is one of the government's Roads of National Significance, and its price tag is now between $1.4bn and $1.9bn.
In response to written questions from the Green Party, Minister of Transport Simon Bridges said the benefit-cost ratio (BCR) is 25 cents for every dollar spent, but is still subject to more detailed analysis.
Green Party transport spokesperson Julie Anne Genter said the rising cost and low benefit figure meant the project should be scrapped, and replaced with safety improvements in areas where they were needed.
"There's almost no question that the benefit number will have already been exaggerated to be as high as possible to support the aims of the government in completing the project," Ms Genter said.
Safety improvements and a bypass could cost around $300 million, and free-up budget to be better spent elsewhere, she said.
Warkworth to Wellsford is the northern half of the Puhoi to Wellsford project which, when it was announced in 2009 by the then Transport Minister, Stephen Joyce, had an estimated price tag of $494m.
The contract to build and maintain the Puhoi to Warkworth section for 25 years has been let for $709.5m
The route for Warkworth to Wellsford has been extended to also bypass the township of Te Hana at it's northern end, and NZTA said it believed the cost could come down when construction tenders were eventually sought.
NZTA said the new northern section of State Highway One would stop serious accidents forcing it to close - something that happens, on average, five times a year - and reduce deaths and serious injuries by 80 percent.
It would also remove 90 percent of through traffic from the town of Wellsford, and cut seven minutes from a typical car journey.
The Automobile Association (AA) backs the project despite the low BCR for the northern section.
Puhoi to Wellsford has a BCR of 1.1, meaning there was $1.10 of benefit for every dollar spent.
"Once you look at the BCR in a whole-of-project sense, it stacks up much more," AA's Principal Advisor Barney Irvine said.
Mr Irvine said for Northland SH1 was a social and economic lifeline and it has really strong support from within the AA's membership because it brings growth opportunities, an increased sense of safety, and better connecting communities.
"The fact of the matter is if the BCR was the only measure, projects all over the country would struggle to get across the line, including walking cycling and roading projects," he said.
Transport Minister Simon Bridges declined to comment to RNZ.
Benefit-cost ratios previously criticised by government
BCRs have also been used by the government in the past as a criticism.
In 2012, the then Minister of Transport Gerry Brownlee criticised the 0.44 BCR for Auckland Council's proposed City Rail Link project.
"With a modelled benefit cost ratio of just 44 cents in the dollar, the benefits of the CRL are nowhere near the cost of building it," Mr Brownlee said at the time in a statement.
Six months later then Prime Minister John Key overturned the government's public scepticism, and agreed to part-fund the downtown rail tunnel, expected to cost about $3bn and now under construction.