The Labour Party says a document obtained by Radio New Zealand casts doubt on the credibility of cost estimates for the Government's roads of national significance programme.
A Transport Agency Board paper from March says sections of four roads of national significance will soon reach a critical milestone in their development where preferred options are identified and priced.
The agency says that will provide the it with the first firm cost estimate.
The roads are the Waikato Expressway, Wellington Northern Corridor, Puhoi to Wellsford and Christchurch highway improvements, which collectively are expected to cost about $7 billion to build.
Labour transport spokesperson Phil Twyford says there is a risk that costs will escalate, and if that happens the country will be left with "dinosaur projects" that are not able to be completed.
But Transport Minister Gerry Brownlee says the estimates are sound. "When we're looking at projects that run out over a 10 year period you've got to look at a number of variables that will occur during that time that might materially affect the real costs of those projects coming down."
In all, seven road projects are designated of national significance and the Government's goal is to complete most of them by the 2020/21 financial year at an estimated $10 billion cost.