Transport Minister Simon Bridges says the Government remains committed to a major motorway north of Auckland despite the projected cost nearly doubling to $1.9 billion.

The Green Party said today that the proposed Warkworth to Wellsford motorway should be scrapped and the funding redirected into better-value projects in Auckland like light rail.

The 27km road - which makes up the northern section of the larger Puhoi to Wellsford motorway project - is still in the planning stages.

When it was announced by former Transport Minister Steven Joyce in 2009, the estimated cost was $494 million. That was revised to $1 billion in 2012.

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Figures released to the Greens showed the cost was now between $1.4 billion and $1.9 billion, though new estimates would be made when the final route was decided.

It would provide benefits of 25c for every $1 invested. However, the Puhoi to Wellsford project as a whole had a benefit-cost ratio of $1.10 for every $1 invested.

Bridges said the Warkworth-Wellsford section was more costly because the planned route was through deep terrain and was difficult to work in.

But the completed project would be a "game changer" for the region, "not just economically but socially", he said.

"Despite what the Greens say this is incredibly popular up north because they know what it will mean for them."

The project would was part of a longer-term plan for an improved transport corridor through to Whangarei, he said.

Genter said the northern section was "eye-wateringly expensive" and the money was better spent in Auckland.

"If we took that $1.9 billion dollars and invested it in mass transit we could put light rail from Wynyard quarter, up Queen Street, and all the way down Dominion Road and still have change left over."

One of the key reasons for upgrading the highway is to make it safer. The existing route includes the Dome Valley, an accident blackspot.

The New Zealand Transport Agency (NZTA) estimates that 19 deaths will be prevented every five years once the motorway is in place - a reduction of 80 per cent.

Genter accepted that there were safety concerns for motorists between Warkworth and Wellsford.

"But it's been estimated that these can be resolved for less than a quarter of the cost of this proposed motorway with simple targeted road widening and barrier installations."

New Zealand First leader and Northland MP Winston Peter said he was more concerned about the slow progress on the Puhoi-Warkworth section.

He said not a single metre of road had been built, nine years after it was promised by the Government.

The NZTA estimates that the Warkworth-Wellsford motorway will cut seven minutes off travel times between Warkworth and Te Hana by 2026.