National Party leader Simon Bridges reckons light rail to the airport in Auckland is so unlikely that he would run down Dominion Rd in his underpants if construction started before 2028.

Bridges has already made a pledge to do such a stunt if construction began before the 2020 election, but this morning he suggested extending the date to 2028.

"I could probably make that promise for 2028 and I'd still be safe," Bridges said when asked about his undies-run pledge this morning.

"There ain't any chance of that [construction starting by the 2020 election]. This Government hasn't done anything when it comes to transport."

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He stopped short of making the 2028 deadline his new promise.

The Government is currently considering two bids for the project: an unsolicited one from NZ Infra, a joint venture between the New Zealand Super Fund and Canada's CDPQ Infra group, and one from the NZ Transport Agency.

Transport Minister Phil Twyford said that the project had been delayed by six to eight months following NZTA's failure to conduct an assessment of the bids after Cabinet asked for one.

Concerns have also been raised about potential cost blow-outs, with Deputy Prime Minister Winston Peters saying yesterday that a credible source in the transport sector had told him to be on the alert for such blowouts.

Labour campaigned in 2017 on building light rail in Auckland from the city to the airport, estimating at the time that it would cost $4b - based on the NZTA proposal.

This morning Prime Minister Jacinda Ardern would not be drawn on reports that the NZ Infra proposal could cost $10b, or whether it was more about heavy rail than light rail.

Bridges, a former Transport Minister, said the NZ Infra proposal was "deeply cynical" and if National won the 2020 election, it would pull out if no contract had been signed.

"They're effectively trying to take [NZ] Super profits off a gullible Government and laugh themselves all the way to the bank.

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"Even on the NZ Infra proposal, they're talking about a construction start time of 2025 ... I'd say we won't see this start until the late 2020s."

He preferred extending heavy rail from Puhinui into the CBD from the airport, which he said would cost about $1b.

He criticised Twyford for taking so long to choose between an urban regeneration rail line with many stops or a rapid transit line with fewer ones.

"All my sources, and I've got many in the Transport sector, that what's actually gone on here is that Phil Twyford has not for two years been able to make his mind up."

Twyford said they were not mutually exclusive, and high-quality rapid transit corridors would see more housing and retail outlets to help regenerate urban areas.

The rail line in the NZ Infra bid would see someone leave the CBD and be at the airport in under 30 minutes, he said.

"That kind of speed on a critical part of the rapid transit network would have a major impact on Auckland's congestion.

"It's about delivering the mobility in our biggest city. It's about moving large numbers of people across the city."