Auckland Transport is considering running trams to the airport via Dominion Rd, one of the Super City's busiest commuter routes.
Its consultants have nominated 20 potential tram stops between the airport and Britomart - including eight along Dominion Rd and four on Queen St - but questions are being raised about how the route could serve both long-distance travellers and commuters.
An estimated journey time of 44 minutes for the 21km trip is seen as optimistic by the Campaign for Better Transport and Auckland Council infrastructure chairman Mike Lee, who heads a steering group of parties including KiwiRail considering "rapid transit" options for the airport.
That time assumes trams would travel at up to 80km/h and compares with about 39 minutes for 110km/h electric trains or up to an hour for buses jostling with other traffic.
Transport campaign spokesman Graeme Easte, an Albert-Eden Local Board member, said yesterday that he feared commuters on what was now a very convenient and frequent-service bus route would lose out.
"People wanting to get home at rush hour will be tripping over people with luggage, large packs and the odd surfboard," he said.
Mr Lee said he had reservations about the effectiveness of a longer-distance rapid transit airport service using Dominion Rd and was sceptical about some of the numbers and assumptions in a report by the consultants.
He believed there was "a major flaw" in business case comparisons estimating the cost of extending trains to the airport at $2.3 billion compared with $1.1 billion for trams.
That was because electric rail already ran to Onehunga - just 10km from the airport - yet a tram line via Dominion Rd "exists only in the imagination at this stage".
Mr Easte said he understood the light-rail estimate was on top of whatever it would cost to run tram lines along Dominion Rd to Denbigh Ave, from where they would turn east to follow the Southwestern Motorway to the airport.
Light rail along key routes - starting with Dominion Rd before including others such as Mt Eden Rd and Sandringham Rd - was added this year to Auckland's regional land transport plan with the aim of stopping the central city from becoming clogged by buses from the early 2020s.
Although funds have yet to be allocated, Auckland Transport chairman Lester Levy said then he did not expect a first stage from Britomart to the southern end of Dominion Rd to exceed $1 billion.
An Auckland Transport spokeswoman said yesterday a decision was likely by the middle of next year on whether the airport should be served by trains or trams.
People wanting to get home at rush hour will be tripping over people with luggage, large packs and the odd surfboard.
She expected the consultants would be guided by "international best practice".
The transport body is under pressure from the airport company to choose between heavy and light rail by then, to fit in with the company's plans to build a new domestic passenger terminal by 2021 and a second runway a few years later.
Airport development chief Graham Matthews says the company is "mode agnostic" between heavy or light rail, but trains will need a wider turning circle to reach its terminals.
"Both would provide significant added capacity for public transport to the airport, which is what we want to achieve," he told the Herald.
But he said whichever mode was chosen, its tracks would have to run under the proposed second runway to an underground train station or surface tram stop.
Although Mayor Len Brown initially promised airport rail by 2020 as his second transport priority after the $2.5 billion underground train extension from Britomart, that has since been relegated to the Auckland Plan's second decade - from 2021 to 2030.