The firm hired by the Government to plan a new billion dollar motorway had earlier claimed the costs outweighed the economic benefits.
The so-called holiday highway, north of Auckland, will stretch from Puhoi to Warkworth.
Four years ago a roading consultancy group now working on the upgrade wrote: "The scope for substantial economic growth...is limited."
The Green Party says this shows the 'road of national significance' isn't very significant at all.
The new and improved highway is part of a $1.7 billion roading upgrade from Auckland to Northland and critics say it's a waste of money.
"The highway really is a holiday highway," Green Party MP Julie Anne Genter said. "It's not serving many motorists or much freight, it's mainly congested on the weekends, so it's really not a good use of almost two billion dollars."
A report obtained by ONE News shows the roading consultancy firm now planning the upgrade originally thought it would have little economic benefit.
In 2008 Sinclair Knight Merz wrote that the project would "have costs exceeding benefits", adding "the scope for substantial economic growth...is limited in the region.
But it said overall the highway would make a "positive contribution".
Genter said projects being funded for economic reasons must be based on evidence, not just politics.
After the National-led Government was elected, it announced it would upgrade the route as part of its roads of national significance package. In late 2009 Sinclair Knight Merz was asked to write another report which critics say has a totally different take on the project.
That report says the upgrade "performs well...against the criteria for strategic investment" and "the benefits of the project...would...offset the low benefit cost ratio."
Genter believes they were told by their clients find a way to justify it.
But Transport Minister Gerry Brownlee said "one of the things you've got to be careful about is relying constantly on costs benefit analysis on road by road."
"The reality is that Northland is a growth region but it's also a somewhat depressed region," the minister said, adding that if Aucklanders were looking at the Harbour Bridge now under the same sort of analysis "it probably would never have been built".
Sinclair Knight Merz won a multi-million dollar contract to plan the upgrade and declined to comment to ONE News - as did the transport agency.
The Green Party says it's not too late for the Government to scrap the project.