Controversial plans to extend Auckland's motorway network to Warkworth have won draft approval after a public hearing over three months.

A Government-appointed board of inquiry yesterday approved in principle the Transport Agency's $760 million motorway extension proposal - derided by critics as a "holiday highway" - over 18.5km of difficult terrain from the Johnstones Hill tunnels to just north of Warkworth.

The decision, although subject to a comments period of 20 working days before it can be confirmed, has come as a relief to the agency after its plans for a road flyover above the Basin Reserve in Wellington were knocked back by another board this week.

Acting northern highway manager Steve Mutton called the draft decision "great news and an exciting and important step" towards improving transport connections between Auckland and Northland.

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But he remained vague about a starting date for the five-year extension project, saying that subject to confirmation of the board's decision in September, it "may" start between 2016 and 2019.

Funding for the project - which could proceed as a public/private partnership - has yet to be determined, although road tolls will be considered to help to pay for it.

The link is just the first stage of a "road of national significance" which the Government wants built to Wellsford, but no timetable has been indicated for the second leg after the agency discovered major geotechnical challenges.

Labour Transport spokesman Phil Twyford said the Warkworth decision did not stack up against an alternative proposal of the Campaign for Better Transport, on which he promises his party in office will spend $320 million to fix accident black spots and congestion pinch-points on the existing State Highway 1, and build a bypass around Warkworth.

"The Government's $760 million plan is a duplicate gold-plated toll road that would only cut three minutes off travelling time between Puhoi and Warkworth, on a road that outside the summer holidays carries less traffic than many suburban arterials," he said.

Green Party transport spokeswoman Julie Anne Genter said the extension would worsen congestion by encouraging more commuting to Auckland by car.

Northland Regional Council transport chairman John Bain welcomed the decision as providing for the first stage of a badly needed upgraded gateway to the north, which he said sent $1.8 billion of goods a year by road to Auckland and beyond.

The five-member board, chaired by retired High Court judge John Priestley, QC, approved most conditions proposed by the Transport Agency for the project, which will include 12 bridges and viaducts and involve the movement of about eight million cubic metres of earth and rock over a route in the Puhoi and Mahurangi River catchments.

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The board said the benefits of the motorway extension over the existing SH1 were "compelling" in terms of safety, travel times and fuel savings, such as by providing an easier route for heavy vehicles now labouring up Schedewys Hill.

Although the agency is staying clear of the Pohuehue Scenic reserve, it has been unable to avoid a narrow bush corridor at the end of Perry Rd further north, where about 374 kauri trees may have to be felled.

But the board has accepted its proposals for replanting native vegetation, and for minimising the risk of kauri dieback disease, such as quarantining earthmoving machinery entering sensitive areas.

On the other hand, the board has rejected a Transport Agency assertion that extra construction traffic will have only minor impacts on Warkworth's difficult turnoff to eastern beaches, and ordered that project vehicles not use it at peak times.