Those politicians closest to Let's Get Wellington Moving are divided over whether a mysterious letter the Associate Transport Minister penned about the project should be released.

The letter Julie Anne Genter sent to Transport Minister Phil Twyford in March this year has heightened speculation over how much influence the Greens had over the $6.4b project.

The letter's existence was revealed through an Official Information Act request response, but Genter has refused to release it for reasons including to maintain the effective conduct of public affairs through the free and frank expression of opinions between Ministers of the Crown.

Twyford agreed it should not be released.

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"Governments do not release correspondence between ministers generally or certainly between coalition and confidence and supply partners. That's a convention of Government, and we don't intend to change that."

It shouldn't come as a surprise there were "robust conversations" as details of the package were worked out, he said.

When put under pressure by the Opposition in the House yesterday, Genter did reveal she expressed concerns in the letter about the lack of sequencing of projects in the $6.4b transport project, including a second Mt Victoria Tunnel.

Transport Minister Phil Twyford agrees the letter should not be released. Photo / Mark Mitchell.
Transport Minister Phil Twyford agrees the letter should not be released. Photo / Mark Mitchell.

Greater Wellington Regional Council Sustainable Transport Committee deputy chair Daran Ponter said the letter was presumably a means by which Genter sought to influence the process.

He said it should be released.

"Let's see the evidence of that. Let's see what it is that she had pushed for and the direction that she has changed, because the things that have arrived on Wellingtonians' plate in relation to Let's Get Welly Moving are certainly not the things that they identified as projects they wanted when it went into the parliamentary process."

The Government endorsed LGWM package has shoved a second Mt Victoria tunnel on the back burner. Meanwhile doubling The Terrace tunnel and trenching Karo Drive didn't make the cut at all.

Greater Wellington Regional Council chair Chris Laidlaw said there was a good case for releasing the letter in the interests of transparency.

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But it was essentially a matter between two parties, he said.

"Coalitions are always bumpy and in many respects the Let's Get Wellington Moving programme of investment is the product of a coalition deal."

Wellington mayor Justin Lester said he wasn't interested in a "political spat" between the Opposition and the Government.

He said Wellingtonians wanted to get on with the project and didn't think they were interested in the letter.

"The letter is absolutely irrelevant now, the decisions have been made."

Wellington mayor Justin Lester says he's not interested in a political spat between the Opposition and the Government. Photo / Mark Mitchell.
Wellington mayor Justin Lester says he's not interested in a political spat between the Opposition and the Government. Photo / Mark Mitchell.

Wellington City Council's transport portfolio leader Chris Calvi-Freeman said the important thing was the Government had made those decisions.

"Exactly how they came to that within the machinations of politics and what happens in the Beehive is not uppermost in my mind."

Calvi-Freeman was not "overly concerned" about the political power the Greens appear to have had over the project.

"Obviously the Greens are a partner in Government and I think they have every right to express their opinions to the Minister."