Buses are a better, cheaper solution to Mt Roskill's traffic problems than light rail, according to Prime Minister John Key.
"Light rail, when they've rolled it out in Australia, has had massive cost blowouts," said Mr Key on the Paul Henry programme this morning.
Andrew Little on Sunday announced plans to build a 13km line from Wynyard Quarter to Britomart, along Queen Street to Dominion Road and ending in Mt Roskill - where a by-election is due to be held in December.
Construction will begin "as soon as possible", and the line will be a foundation for a light rail network in Auckland.
But Mr Key said the agreement between the Government, Auckland Council and Auckland Transport is to "potentially" build a light rail system in 20 years' time - not now.
"It was only about a week or so ago [Auckland Mayor and former Labour MP for Mt Roskill] Phil Goff was saying [the council has] got no money - now Labour is saying they'd have to pony up with $700 million of ratepayers' money for a by-election Labour wants to win."
The project's estimated to cost $1.3 billion, with half coming from taxpayers and the rest from Auckland Council.
"This project is already in the Government's plans," Labour's associate transport spokesman Phil Twyford told Paul Henry. "It's very funny that the Prime Minister comes on your show and wants to rubbish it - it's his policy. We're just bringing it forward. It's in the plan."
Mr Twyford said the line would be built "as soon as it can be financed" as the party considers Auckland's congestion issues an urgent priority, and Mt Roskill is currently a "void" when it comes to public transport.
He said Dominion Rd and Sandringham Rd were designed to carry rail, and did right up until the 1950s.
But while National's Mt Roskill candidate Parmjeet Parmar agrees the suburb needs more public transport, she thinks more buses and T3 lanes a better solution.
And at least one academic has called Labour's promise an "obviously political move", whose timing is no coincidence.
Massey University Associate Professor Grant Duncan said the light rail plan is a vote-winner, but it shouldn't stop at Mt Roskill.
"It rather reminds me of the student loans policy back at the 2005 election campaign, where it's pretty much gazumped National. It's one of those trump cards that Labour is playing, and in all likelihood it may very well work for them."
Prof Duncan said the line should extend through to Mangere and the airport.
Mr Key admitted his party is fighting an uphill battle to win the December 3 by-election.
"No incumbent government's won a seat they didn't hold," he told Paul Henry. "History doesn't argue strongly for [us]."
Mr Goff held the Mt Roskill seat since 1999. Before that he held the now-defunct Roskill seat, and briefly, New Lynn.
"We'd love to win it, but we're realistic about our chances," said Mr Key.