Work-hungry roading contractors are lining up for a $2 billion construction bonanza to complete Auckland's western ring route between Manukau and Albany.

The Government's Transport Agency announced yesterday that its board had approved up to that amount to extend State Highway 20 through Waterview.

The job will include digging twin tunnels for 2.4km of the 4.5km link, and widening the Northwestern Motorway between St Lukes and Westgate.

The agency drew praise from the construction industry - and criticism from the Tunnel or Nothing community group - for deciding to begin a tendering process for the $1 billion-plus Waterview connection project before applying for resource consents and land designations.

A call for registrations of interest will be advertised on Government and international websites from today, although the agency and Transport Minister Steven Joyce say no construction work will begin on the main project until approvals are obtained through a new streamlined national consenting process.

The agency intends applying to the Environmental Protection Authority for consents next month, in the hope of receiving a favourable decision from a board of inquiry by July next year, after a "one-stop" hearing in March or April.

Chief executive Geoff Dangerfield, accompanied by Mr Joyce for yesterday's announcements, produced an "indicative" timetable including a construction start of October next year although he acknowledged possible appeals limited to points of law could alter that.

A combined meeting of the Auckland City Council's transport and arts, recreation and culture committees will also be held in a fortnight to hear final submissions from community representatives before approving a list of mitigation measures to be sought through the consenting process.

Tunnel or Nothing spokeswoman Margi Watson, a Waterview resident whose organisation wants tunnels to run the full length of the route, questioned how tenders could be called "when there is no evidence of true mitigation or good legal process".

She accused Mr Joyce of "creating undue influence over the decision-making process" by sanctioning the early call.

"We are still waiting to see mitigation or minimisation of the severe negative impacts on the environment and communities along its [the motorway's] path."

Mr Dangerfield promised to work closely with the community and local authorities to mitigate the effects of the project, but said it was so large and complex that his organisation needed to ensure both the market capacity and capability existed to build the tunnels.

He expected time and financial savings from running the tendering and consenting processes in tandem.

Mr Joyce said completion of the 48km ring route was the most crucial transport project for securing greater productivity and economic growth in New Zealand's largest city, and it was important to start the tendering process as soon as possible to take advantage of a favourable construction cycle.

The completed route would create 2000 jobs and give Auckland a fitting entrance from its airport, from where visitors and businesspeople would be able to go downtown via Waterview.

Asked whether an airport rail link would be similarly inviting, he said the Government wanted to assess the impact of its existing $1.6 billion upgrade to the region's rail network before considering future projects.

Roading New Zealand chief executive Chris Olsen welcomed the early tendering, saying jobs had been lost in his industry in the past year and it was important to restore confidence to prevent a continuing loss of skills and resources to Australia, which was gearing up for a giant construction boom.

He said local roading contractors had already teamed up with engineering consultants and international tunnelling companies to form consortia "for this iconic New Zealand engineering project".

Transport Agency regional highways manager Tommy Parker said the project was so big that it might help reverse the cross-Tasman flow.

He was aware of "at least two or three" teams which were already well-advanced, with international tunnelling experts on board.

A short-list of two leading teams, each with agency representation, would be drawn up after Christmas on the basis of the most attractive design, cost and methodology package and the winner would simply "continue straight into the contract".