The Minister of Transport's attempts to block an Official Information Act request are now under investigation.

Simon Bridges is in hot water for pressuring KiwiRail to not release information about a proposed railway for Auckland.

Chief Ombudsman Peter Boshier says he's received several complaints about the move.

"Now that we've been asked to investigate, I want there to be an understanding that we'll do this promptly, we'll do it assiduously and we'll deliver a result which is timely."

Mr Boshier says no minister has the right to interfere with the Act and agencies should be able to make their own rulings unobstructed.

Mr Bridges is standing by attempts by his office to block the request.

"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'," he told The Nation on Saturday.

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.

Newshub.