Auckland Transport is having to stump of $21 million to "future-proof" a motorway project for trains or trams to the airport.

The council body is paying the money to the Government's Transport Agency, towards extra costs of designing a motorway interchange for trams to run through a trench beneath Kirkbride Road, Mangere, or trains on an elevated line.

That is additional to $140m the agency is spending on the 580-metre trench and a motorway extension to the airport in an accelerated Government-funded project.

Preparations are well-advanced for the trench to be dug west of George Bolt Drive.

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The extra cost was revealed to a community liaison meeting at which Campaign for Better Transport representative Graeme Easte was shocked by a map showing no spare room in a highway designation between the trench and the airport for a railway.

"It showed the entire designation taken up by the motorway and a cycleway," he said.

"But cycling is lower priority than high volume rapid transit."

Auckland Transport project director Theunis van Schalkwyk has since, in a joint statement with the Transport Agency to the Herald, confirmed that his organisation has allocated $21m to make the trench 3.5m wider than planned.

Its new width of 29m would provide an 8m rail corridor, which the statement said would be enough for trams to run through the trench or for elevated trains above it.

The statement also confirmed the motorway extension between the trench and the airport would use up "every part" of an existing road designation through pinch-point zones to a width of 28.4m to accommodate bus lanes and a shared walking and cycling path.

But Mr van Schalkwyk said Auckland Transport's budget also allowed $12.6m for route protection.

"We're currently developing a business case considering both heavy and light rail route options for rail to Mangere and the airport employment area," Mr van Schalkwyk said.

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"This includes their differing benefits, costs and potential impacts on property, including other requirements in the motorway corridor beyond the Kirkbride interchange."

A report would go to Auckland Transport's board by the end of the year.

Transport Agency regional director Ernst Zollner said his organisation was "extremely committed to providing a rail link connecting the airport and the city."

That included ensuring a rail extension from Onehunga could go over another interchange to be designed in the next three years to connect a new east-west motorway from Penrose to State Highway 20 north of Manukau Harbour.

Campaign for Better Transport convenor Cameron Pitches was reassured by the statement but said that if Auckland Transport had acted sooner on a petition for which his group collected 10,000 signatures in 2007 calling for airport rail, it may have avoided the extra cost.

He did not share Mr Easte's concern about a bikeway taking precedence over rail, saying there should be provision for both modes, although a failure to protect a route until now may mean a costly underground connection to the airport.

Auckland Council infrastructure chairman Mike Lee is furious at the "snail's pace" of Auckland Transport's management, and its resurrection of trams as an option after a steering group he chaired ruled those out in 2011 in favour of fast electric trains to the airport.

He rejects a cost estimate by Auckland Transport last year of $1.63 billion for heavy rail from Onehunga to the airport by 2045, given the distance is just 10km.

Airport rail:

• Auckland Transport estimates a railway extension carrying electric trains from Onehunga to the airport by 2045 would cost $1.63 billion.
• Auckland Council infrastructure chair Mike Lee rejects the figure as "imaginary" for a link of just 10km, and is furious the transport planners have resurrected trams as an option.