The new estimate appears to have prompted Auckland Transport to reconsider light rail - or trams - to the airport to cut costs even though the multi-agency group rejected it.
"Light rail would provide more opportunities to route the track in a way that reduces the infrastructure cost," a spokesman told the Herald.
"A light rail corridor could be routed in a way that removes substantial costs associated with placing heavy rail in a trench."
The development has dismayed Mike Lee, Auckland Council's infrastructure committee chairman, who chaired the multi-agency group, as well as the Campaign for Better Transport, which sponsored a 10,000-signature petition in 2007 calling for airport rail.
Mr Brown noted from China last night the group's support for trains to the airport as the best long-term solution and said: "Aucklanders would need to be convinced any other option would stack up." But a route should be "future-proofed" for either option.
Mr Lee said light rail was suitable for inner-city transit but not for long-haul work. "Unless you're going to build 23km of new tram tracks winding through the suburbs, passengers would have to disembark [with their] baggage at Onehunga to wait for a train."
Cameron Pitches of the Campaign for Better Transport said it was frustrating costs were climbing as land was being built on rapidly in the path of a potential rail route.