Transport Minister Simon Bridges has been caught trying to block an official information request for details about a proposed new $50 million Auckland railway line.

Photo: VNP / Daniela Maoate-Cox

New Zealand First leader Winston Peters tabled an email trail in Parliament yesterday showing that Mr Bridges' office repeatedly urged KiwiRail last week not to release a business case on Auckland's proposed third main railway track.

Initially, his officials opposed the document being released, saying it was part of an unsuccessful budget bid, but were told by KiwiRail on Thursday that the law was clear it should be released.

After consulting its legal team, KiwiRail told Mr Bridge's office it would struggle to justify not releasing it.

But on Friday Mr Bridges' office again urged KiwiRail not to release the business plan.

This time it used a scatter-gun approach - arguing the report was only a draft, was on a misleading template and that its proposed release was making them "extremely uncomfortable".

Mr Peters said Mr Bridges was clearly been trying to heavy KiwiRail into keeping the information secret.

"You've got the minister after nine years of government exhibiting all the arrogance that often long-time governments do exhibit - and he's saying 'well we'll be seriously uncomfortable about that'.

"Well the Official Information Act is not about the comfort zone of governments or ministers, it's about the right of the public and the media to have critical information that they need," Mr Peters said.

The business case for the third main track had been requested under the Official Information Act by public transport advocate group Greater Auckland.

Writer Harriet Gale said the proposed railway line was seen as crucial in helping KiwiRail manage freight train movements while passenger services were booming.

"This is a really integral piece of infrastructure, we're talking $50 million really, it's a small part of any transportation budget," she said.

"It's mostly completed - KiwiRail just needs the last $50 million to complete it."

Ms Gale said KiwiRail made it clear the business case did not need to be kept secret and that the minister's behaviour was worrying.

"I think what's disappointing overall is that it's just a general tendency in this process of gathering official information that we're just seeing so many rejections and so many redactions in official information.

"And we've seen this because New Zealand has dropped in press freedom indexes this year as a result of the way we are dealing with official information."

Mr Bridges refused to be interviewed but in a statement said his office had only offered a view on the proposed release of information and said it was up to KiwiRail whether they did so or not.