New Zealand First leader Winston Peters says he has been told to be "alert" to potential cost blowouts for the Government's flagship light rail project for Auckland.

But Peters, who is also Deputy Prime Minister, said he has had no such concerns raised with him from ministerial colleagues, including Transport Minister Phil Twyford.

Labour campaigned in 2017 on building light rail in Auckland and its confidence and supply agreement with the Green Party promises that "work will begin on light rail from the city to the airport in Auckland".

But there is nothing about the project in the Labour-NZ First coalition agreement.

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New Zealand First could potentially put a stop to the project if it didn't think the business case stacked up.

Stuff reported that Associate Transport Minister Shane Jones, who is also a NZ First MP, was horrified by some projections of costings for the project, including warnings that those costs could rise.

Peters said he had not had advice from any minister about potential cost blowouts, but he had been warned by a source in the transport industry "for some time".

"If I've had cause to say, 'Prick up your ears and try and listen and find out,' yes I have been told to be on the alert for costs," Peters said this morning.

"Mr Jones wasn't my cause for being concerned."

He suggested that a cost blowout for a project currently estimated to cost $6 billion could be a reason for NZ First to oppose the project.

"A blowout is something that every politician should be looking to avoid.

"We'd have to know that there wasn't a blowout, going into the future."

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Twyford said there could not be a cost blowout because the Government was yet to decide on a confirmed project or builder.

The Government is currently considering two bids for the project: 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.

"Cabinet – which NZ First is very much a part of – agreed to a market process to find the best option to build light rail in Auckland," Twyford said in a statement.

Transport Minister Phil Twyford says there has been no cost blowout because no contract has been awarded yet. Photo / Mark Mitchell
Transport Minister Phil Twyford says there has been no cost blowout because no contract has been awarded yet. Photo / Mark Mitchell

He rejected a report in Stuff that the NZ Infra bid was tantamount to a back-of-the-envelope proposal.

"The Fairfax story is factually incorrect. It is ludicrous to suggest the Government didn't properly investigate the NZ Infra unsolicited bid.

"A quick look at Cabinet papers proactively released by the Ministry of Transport shows the extensive assessment that was done.

"Officials went to Canada to look at the light rail work the NZ Super Fund's Canadian partners are doing in Montreal."

Stuff quoted a letter from interim NZTA board chairman Nick Rogers, who said the NZ Infra bid was "vague" and "little more than an idea set out on six pages of power point presentation".

Twyford said that Rogers had written the letter wanting NZTA to win the contract.

"The NZ Infra proposal was progressed following a rigorous investigation into its merits," Twyford said.

"At the same time the Transport Agency is preparing its own proposal and Cabinet will decide which is the best option for Aucklanders and for New Zealand taxpayers in February."

Prime Minister Jacinda Ardern this morning said the Government was committed to the light rail project.