KiwiRail is refusing to reveal how its top officials reacted after the Transport Minister's office urged them to keep a report secret.
KiwiRail caved in to pressure from Simon Bridge's office, which didn't want the business case for a proposed Auckland rail line to be released, despite initially insisting the document legally had to be made public.
Mr Bridges' staff complained to a number of senior officials at KiwiRail saying the minister was extremely uncomfortable with the report's proposed release. KiwiRail then decided to withhold it.
The state-owned enterprise has declined to release the emails of those officials on the matter, saying it needs to "maintain the effective conduct of public affairs".
RNZ has complained to the Ombudsman.
Chief Ombudsman Peter Boshier wrote to Prime Minister Bill English after the story broke last month, saying "such incidents have the potential to erode public confidence in democracy and the government".
After the role Mr Bridges played was revealed in Parliament by New Zealand First leader Winston Peters, KiwiRail changed its mind again and released part of the Third Main Line business case report.