The Transport Minister is standing by office's attempts to block an Official Information Act request.
Emails from Simon Bridges' office were leaked in early June, revealing that KiwiRail officials were put under pressure to withhold a business case regarding a proposed third main railway track in Auckland.
Mr Bridges told The Nation he had good reason to try to keep it under his hat.
"There are really strong grounds, over a long period of time in terms of convention, practice and the law, as to why my office I think was right to say 'no, this shouldn't go out'.
NZ First leader Winston Peters revealed the cover-up in Parliament, and tabled what he said was a summary of emails.
"Simon Bridges' office deliberately tried to influence officials, behind closed doors, covertly and secretly, not to release information that is owed to the public of this country," he said.
Public transport advocacy group Greater Auckland had asked for the information. They say the proposed third rail line was seen as crucial, so KiwiRail could move freight as passenger services are growing.
But Mr Bridges says the case was an early bid, the numbers were wrong and he wasn't comfortable with it.
"In terms of its numbers, in terms of much of what it said, I happen, as I say, to be in favour of the project in general."
If it goes ahead, the project is estimated to cost $58 million.