Julie Anne Genter's justification for refusing to release letter to Phil Twyford

Julie Anne Genter is continuing to refuse to publicly release a letter she wrote to the Minister of Transport despite an official investigation being launched.

National MPs are calling for the release of the letter over concerns the Greens influenced a decision to delay the proposed second Mt Victoria tunnel. 

Genter, a Green MP and the Associate Minister of Transport, has confirmed she wrote a letter to Transport Minister Phil Twyford on 26 March about the Let's Get Wellington Moving (LGWM) transport package.

Announced in May, the $6.4 billion package proposes integrating modern rapid transit, walking and cycling upgrades, and an extra Mt Victoria tunnel set for completion between 2030 and 2034.

National's transport spokesperson Chris Bishop has raised concerns about the proposed completion date of the tunnel. He's alleged Genter convinced Twyford to push back the date over her concerns about it.

She confirmed in Parliament late last month that a letter she sent to Twyford weeks before the package was announced related to "concerns" about the sequencing of projects, particularly the second tunnel.

The original LGWM proposal - a joint initiative between Wellington City Council, Greater Wellington Regional Council and the NZ Transport Agency - planned for a second Mt Victoria tunnel be built between 2024 and 2029.

But the LGWM proposal announced by the Government after it agreed to adopt the plan had the completion date of the second Mt Victoria Tunnel pushed back to between 2030 and 2034.

Bishop has said Wellingtonians deserve to know how much Genter was responsible for pushing back plans for a second Mt Victoria Tunnel to beyond 2030.

He's asked the minister to release the letter she sent to Twyford, but she's refused, claiming that because she is a Green MP she isn't subject to the public disclosure requirements normally faced by ministers.

Genter said in Parliament on Wednesday that while she is the Associate Minister of Transport, she is also the Green Party spokesperson on transport issues.

She said she has been going off advice "that we should not release the letter" which she said was in line with the view of the Chief Ombudsman Peter Boshier in June - a ruling he made public this week.

Genter said the content of the letter was "political party consultation and that is why we consider it is not in the public interest to release it".

This was reflected by Boshier who said there was "strong interest in maintaining the Government's ability to undertake effective and efficient political consultation with political parties".

"It is vital in this environment that these parties are able to conduct free and frank discussions in a relationship of trust and confidence, and in a timely manner."

Twyford told media on Wednesday that Genter was "writing as the Associate Minister but expressing her view on behalf of the Green Party".

"We talk about things all the time. It shouldn't surprise anyone that we had robust discussions going on as we put together a $6.4 billion transport project."

Twyford said it's "the right thing to do" not to release the letter.

 "If you're going to release all the ministerial consultation that goes on before a paper goes to Cabinet, where differences of opinion are argued out, it would undermine collective responsibility."

National MPs aren't accepting Genter's justification for not releasing the letter. 

Wellington-based MP Nicola Willis has laid a complaint with the chief ombudsman who she said has agreed to investigate whether the letter can continue to be withheld under the Official Information Act.

"Obviously she and Phil Twyford have something to hide, otherwise they would have released the letter by now," Willis said on Tuesday.

"Her confusion shows an appalling lack of understanding of the public accountability that comes with her ministerial warrant."