Official advice from the Infrastructure Commission shows the government could be set to spend 10 percent of its total budget for new infrastructure for the next 25 years on one roading project.

The project "Accelerating Northland Expressway" is a four-lane expressway between Auckland and Whangārei, which the government has agreed in principle to speed up as part of its Roads of National Significance programme.

The commission's advice was sent to infrastructure minister Chris Bishop, in order for him to provide feedback to the transport minister Simeon Brown, and was obtained under the OIA by climate campaign group 350 Aotearoa.

Sent in early July, the advice document reads: "Based on historic annual investment by central government and Treasury's projections of future GDP, we estimate this project alone could consume 10 percent of the total non-maintenance/renewal investment for the next 25 years across all types of central government infrastructure (roads, hospitals, schools, defence, justice, public admin, etc)."

It said it saw a risk that the plan prioritised speed ahead of other considerations, and that "in doing so it may not deliver the pace or outcomes expected".

On 23 July, transport minister Simeon Brown announced plans to speed up the construction of the expressway from Warkworth to Whangārei by combining three separate roading plans into one mega-project, to be built as a public-private partnership.

The project's cost has yet to be estimated.

An investment case will be considered by Cabinet later this year.

The commission also says the paper "should be more transparent about the uncertainty of current cost estimates" - including the risks posed by the difficult terrain the highways would traverse.

"The actual costs could be much higher, and it would not be abnormal for projects of this complexity to cost 50 percent - 100 percent more than initial estimates."

It said it was concerned about the process by which the project had been "identified as being of such high priority for delivery, amid scarce funding".

The commission confirmed on Monday it was "unaware of any changes to Treasury's projections of future GDP or other matters" that would materially alter its estimate, or its advice.

Brown said the paper was intended to "test the market for firms interested in delivering the Northland Expressway", a process which was currently underway.

Officials were due to report back in the coming months, and the government was committed to the four-lane highway.

Bishop declined to comment.

350 Aotearoa campaigner Adam Currie said it showed the government's priorities were "all wrong".

"Is this one motorway really worth one-tenth of our entire country's spending on schools, hospitals, houses and public transport infrastructure?" he asked.

He said bigger roads caused more climate pollution, and the funds would be better spent on other projects.

"The eye-watering cost of this project shows the funds our communities need are there - but the government has their priorities all wrong."