Requests for information made under the Official Information Act (OIA) can be declined for a number of reasons including to enable a minister or department to carry on, without prejudice or disadvantage, negotiations.
In an email in June, KiwiRail told Bridges' office its legal team had advised the business case could not be withheld for this reason.
"Our advice is that releasing the Third Main Business Case would be unlikely to prejudice our negotiations with the Crown for funding for it in future years and prejudice to such an extent does not override any public interest in the release of the information," the email reads.
In an email the next day, Bridges' private secretary emailed an official to raise "a few concerns" about KiwiRail's proposal to release the document in full, and outlining reasons why that should not happen.
"We are still extremely uncomfortable with the idea of releasing this document ... we would like to resolve this before advising the Minister."
Peters said that approach was "yet another symptom of the National Government's arrogance after nine years".
"The Prime Minister has described Alfred Ngaro's comments around using strong-arm tactics on non-profit organisations as naive, and Minister Steven Joyce has claimed 'it's not the way we operate'.
"But what does the Prime Minister have to say about Simon Bridges' office deliberately trying to influence officials behind closed doors to not release information to the public?"
Bridges stood by his office's actions, saying it is normal practice for state-owned enterprises to consult with ministers on the release of information.
"My office offered a view on the proposed release of the information. Ultimately it's up to KiwiRail whether or not they choose to release the information in question.
"I reject Winston Peters' claims that my office has acted inappropriately in its engagement with KiwiRail on this matter."
There have been growing concerns about whether officials are correctly declining to release information that might cause controversy or embarrassment.
Those decisions can be reviewed by the Ombudsman, but that process can take many months because of the large number of complaints.
As a result, there have been calls - including from former Prime Minister Sir Geoffrey Palmer - for a review of the law, or beefed-up resources for the Ombudsman's office.
Sir Geoffrey told a meeting of Transparency International New Zealand last month a strengthened Act would increase protection against corruption and questionable decision-making in central and local government.
In December 2015, then Chief Ombudsman Dame Beverley Wakem said her office's review of the OIA had not discovered malicious or deliberate misuse of the Act by ministers' offices or by government departments.
