Government ministers have been forced to release more details from a letter they've refused to make public, regarding Wellington's long term transport plan.

Photo: RNZ / Dom Thomas

But the Chief Ombudsman has backed the decision of associate minister of transport Julie Anne Genter not to make the letter itself public, saying he accepts her arguments about protecting free and frank communications between ministers, in this case with Minister of Transport Phil Twyford

National has been pushing for the letter to be released, claiming it will show Ms Genter, a Green MP, pressured the minister to delay a proposed second Mount Victoria Tunnel.

Chief Ombudsman Peter Boshier has struck a compromise of sorts, while agreeing the letter need not be made public, he found "overriding public interest" demands the release of some of the information and the context, "to inform public understanding and promote public trust and confidence".

Photo: RNZ/Zoe George

The extra information, provided in a joint statement from the two ministers, confirms Ms Genter wrote to Mr Twyford with her concerns about the indicative transport package, including "inducing traffic and the resulting increased congestion, greenhouse gas emissions and undermining of demand for public transport".

"The Associate Minister was also concerned to ensure there was sufficient funding available for public transport," the statement said.

It cites Ms Genter as advising Mr Twyford "she was comfortable supporting this package if a number of matters were clarified", including that "the public transport, and walking and cycling components of the package would be completed as soon as practicably possible and that work on rapid transit be prioritised ahead of the second Mount Victoria tunnel".

The statement said the letter did not issue any "ultimatums nor threaten a resignation".

National Party transport spokesperson Chris Bishop said this proved what his party had been saying all along.

Photo: RNZ / Richard Tindiller

"She [Julie Anne Genter] should just be up front about the fact the Green Party exerted influence to make sure the second Mount Vic tunnel has been pushed to 2030 or beyond and rapid transit comes first," Mr Bishop said.

"I don't know why she just doesn't accept that and admit it and release the letter."

Mr Boshier also criticised the way Ms Genter handled the controversy, saying statements she made in the House about whether she wrote the letter in her capacity as associate minister versus the Greens' transport spokesperson, "created confusion".

In recent months Ms Genter has said she did raise concerns about the "sequencing of the projects", but would not give further details.

The Ombudsman said those comments were "somewhat vague".

"Clarity is important when the information relates to the policy development of an initiative that, as the complainants note, affects a large number of people and involves a significant amount of public money," Mr Boshier said.

"The statements in the House generated significant public and parliamentary debate, as well as confusion, public disquiet and speculation.

In the joint statement from the ministers, Mr Twyford said in a coalition government "we discuss all major policy decisions with our support parties, and Let's Get Wellington Moving was no different".

"Successive governments have recognised discussions on Cabinet decisions need to take place in confidence to ensure the best decision is made", he said.

"We supported releasing a statement to put to rest the false speculation whipped up by opposition MPs about the letter's contents," Ms Genter said.