The National Party is defending its cost estimates for a proposed four-lane highway from Whangārei to Tauranga, and rejecting criticism it would drive up emissions.

If elected in October, the party says it will build the road - split into four different projects - at a cost of $6 billion. The money will come from the National Land Transport Fund (NLTF) and government investment, with no increase in excise taxes.

The projects, spaced out over a decade, would link Whangārei and Port Marsden, Warkworth and Wellsford, and further south from Cambridge to Piarere, and on State Highway 29 to Tauriko West.

"The reality is that about half the population lives in the upper North Island and it's critically important that we invest in high-quality modern roads which are safe and which connect our region, so that we can drive economic growth and opportunity," National transport spokesperson Simeon Brown told RNZ's Morning Report on Monday.

"I think most New Zealanders who have driven the Waikato Expressway can see the benefits that is provided to our country economically, socially and also from a safety perspective, and what we're saying is we think that the vision needs to continue north to Whangārei, and also south towards Tauranga to connect the, the upper North Island."

Brown says the cited costs were based on "the most recent estimates, which are publicly available we've received from the minister through various information requests".

1 News on Sunday reported documents showed officials' cost estimates were much higher than what National claimed - such as the Warkworth to Wellsford section, which National says would cost $2.2b but officials put at up to $4b.

"We've taken the most conservative options in regards to those, understanding that costs are to increase over time," Brown says.

"We'll also set aside a contingency as well because we think it's important that we're making sure that there's money available to help with any cost increases."

Transport Minister David Parker says National's figures were "laughable at best".

"The cost will be many hundreds of millions, if not billions of dollars more than they are admitting," he told 1 News, calling National's figures "breathtakingly misleading".

The $6b cost National cited would be almost as much as the government planned to spend on state highways nationwide between 2021 and 2014 ($6.6b) through the NLTF. It was larger than planned spending from the NLTF on public transport over that same period ($4.b) and dwarfs the $1b allocated to walking and cycling improvements.

Brown says some of the cost could also be covered by toll roads.

"Ultimately, there's significant benefit economically for people who use them.

"If you think about the Hamilton bypass, you think about the Waikato Expressway, those are having significant economic benefits - not suggesting going back and tolling roads which are already built, but where there are new roads, we're very open to tolling them because ultimately those who benefit from them should be contributing towards them.

"I think most New Zealanders would accept that that's a price they'd be prepared to pay."

Neither the Hamilton bypass nor the expressway were tolled.

The party also wants to scrap the $7.4b Let's Get Wellington Moving project, including light rail, but keep the planned second tunnel under Mt Victoria and add a four-lane highway to the airport.

Let's Get Wellington Moving was a joint effort between Wellington City Council, Greater Wellington Regional Council and Waka Kotahi. National's revised plan it says would cost $4b.

"Let's Get Wellington Moving has become a sort of a joke word in Wellington," Brown says.

"The only thing they've actually built is a pedestrian crossing on Cobham Drive costing $2.5m, and they've spent six years and they haven't been able to actually get Wellington moving at all."

Brown rejected criticism the party's focus on roads over rail would hold back efforts to reduce emissions.

"What ultimately drives on those roads is going to change dramatically over the coming decades. We're gonna have hydrogen trucks, we're going to have electric cars in much greater numbers.

"These things are coming, they're happening already. Electric buses are in all of our major cities through the public transport networks already.

"But ultimately, all of those vehicle movements require roads."

- RNZ.