Key Points:

Transit NZ is finally opening its door to a rail link across Manukau Harbour to Auckland Airport, after years of lobbying by public transport advocates.

The roading agency - which wants to build a duplicate motorway harbour crossing for up to $330 million in time for the 2011 Rugby World Cup - has told the Auckland Regional Transport Authority it is "more than willing" to work with its transport partners to develop an integrated road-rail project providing they raise the necessary extra money.

It has in a letter accepted that building foundations and piers able to support both a motorway and rail crossing would be "the most cost-effective solution".

But it wants the authority and its Auckland Regional Council parent body to stump up the extra $9 million to $10 million needed to make these strong enough.

Transit's transport planning general manager, Wayne McDonald, also highlighted a need for a speedy resolution "as completion of the project by 2011 is seen as a high priority" by many stakeholders.

His agency initially said it would build a duplicate bridge for road traffic only - on a gradient too steep for trains - and was prepared only to ensure the design left enough room for a separate rail crossing to be constructed alongside if and when desired at some later date.

That would have meant stand-alone foundations for each bridge.

But the regional council, in support of years of lobbying by its chairman Mike Lee and the Campaign for Better Transport, complained in a submission to a looming planning hearing that Transit's studies focused too narrowly on economic considerations rather than the environmental impact of separate structures.

The council's transport policy committee yesterday welcomed Transit's concession, but voted for a joint funding formula to be worked out between all parties.

Committee member Bill Burrill expressed reservations, given that a preferred route for a possible rail link to the airport had yet to be determined or justified. But the council's transport group manager, Don Houghton, said the regional transport authority was commissioning a major study to be completed in the second half of this year to select the best rapid transit route to the airport - whether from the north or east, or for trains or buses.

Mr Lee was away from Auckland during the meeting but told the Herald later that Transit was to be commended for "a major step forward."