The Associate Transport Minister says the Government has nothing to hide, but she still won't release a letter she penned when it was crunch time for a decision on Let's Get Wellington Moving.

Under pressure in the House today Julie Anne Genter did however reveal some details of the nature of the letter.

She said she expressed her concerns about the lack of sequencing of projects in the $6.4b transport project, including a second Mt Victoria Tunnel.

The letter sent on March 26 of this year to Transport Minister Phil Twyford has stirred speculation over just how much political power the Greens exerted over the project behind the scenes.

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.

The same day Twyford received Genter's letter he received advice on whether the phasing of projects allowed public transport, walking and cycling ones to happen first.

The following week he received advice on the potential downsides of delaying or removing a second Mt Victoria tunnel.

The existence of the letter was only revealed through an Official Information Act request response.

It raised the eyebrows of local National Party MPs considering both Twyford and Genter's responses to written parliamentary questions suggested there was no correspondence between them over LGWM.

During Question Time today National's Transport spokesman Chris Bishop asked Genter if the reason she didn't disclose the existence of the letter when first asked was because she was embarrassed by its contents.

Genter rejected that and said it was a mistake. As soon as the oversight was realised, the answers were corrected, she said.

Asked whether she would ever release the letter Genter said she could "assure all members that the Government has nothing to hide" and that it was withheld in line with the OIA.

She again referenced reasons including to maintain the effective conduct of public affairs through the free and frank expression of opinions between Ministers of the Crown.

Bishop did clear up one thing, the letter did not refer to "car fascists".

The day LGWM was announced Genter joined discussions on Twitter posting the comment "we need a few car fascists to stop opposing infrastructure that gives more people the option to walk, cycle or scoot safely if they wish."

Genter later acknowledged that wasn't her best choice of words.