Transport Minister Simon Bridges has been cleared of trying to improperly influence officials into keeping a report secret.
However the Ombudsman has criticised KiwiRail, saying the state agency appeared ready to roll over when Mr Bridges' staff asked it to keep a report secret.
Emails released last month revealed KiwiRail reversed a decision to release the report after the minister's office raised concerns with one of its executives.
New Zealand First leader Winston Peters earlier released related correspondence in Parliament and accused Mr Bridges of trying to covertly influence officials behind closed doors.
Chief Ombudsman Peter Boshier said while the Minister's actions fell outside of his investigation's purview, he found "no evidence of improper ministerial involvement".
The Minister's staff were entitled to act as they did, he said.
"There is a line between ministerial input into an agency's OIA decision and ministerial interference, but that line was not crossed in this case."
However Mr Boshier expressed concern KiwiRail appeared willing to change its mind without properly understanding the Minister's concerns and whether they were valid.
"I expect agencies to critically evaluate such concerns."
That was "particularly so" in this case as some of the information was already in the public domain and there was no concern about any commercial impact, he said.
Mr Boshier also noted that senior staff at KiwiRail had previously expressed they would "struggle" to withhold the document on the basis it could prejudice negotiations.
He was also critical of KiwiRail's decision to later release only parts of the document, saying the redactions were rushed and didn't appear "robustly reasoned".
The document - about a proposed rail freight line in Auckland - has now been released in full on KiwiRail's website.
KiwiRail should improve its OIA practices and offer training to its staff, he said.
Mr Boshier also called on all Ministers and state agencies to adopt formal, public guidelines over the way such requests are handled.
"I think this would protect against the perception of impropriety in the processing of OIA requests, which can be just as damaging as the reality of it."
He published a model protocol as an example for agencies and Ministers to use or adapt.
"The clauses in this model agreement include a provision that the agency will consider the views of the Minister with good faith and an open mind before making a decision on a request.
"It also makes clear that the office of a Minister will not provide inappropriate input like asking or instructing the agency to act in a way contrary to the requirements of the OIA."
In a statement, KiwiRail said it followed "usual process" when dealing with the OIA request, seeking further information from its lawyers and the Minister's office before making a decision.
It said it accepted the Ombudsman's recommendations and had begun a review of its OIA processes.
29 May - KiwiRail tells Mr Bridges' staff it intends to release the report.
30 May - Mr Bridges' staff "strongly recommend" the report be withheld, arguing it could form part of a future budget bid.
31 May - KiwiRail consults its legal team.
1 June - KiwiRail tells Mr Bridges' staff it would to "struggle to justify non-release". KiwiRail says its legal advice is that releasing the report "would be unlikely to prejudice our negotiations".
2 June - Mr Bridges' staff escalate the matter to KiwiRail's executive and say they're "extremely uncomfortable" with the report being released.
6 June 1.22pm - KiwiRail revises its response, this time withholding the report in full. "Disclosing the document could inhibit KiwiRail from carrying out, without prejudice, future negotiations."
6 June 1.25pm - Mr Bridges' staff replies: "The Office is supportive of this response."
6 June 3.02pm - Winston Peters raises the matter in Parliament and releases an email trail between Mr Bridges' staff and KiwiRail.
7 June - In a statement, Mr Bridges says his office had only offered a view and it was up to KiwiRail whether they released information.
8 June - Mr Bridges defends his officials' actions and says they were right. Mr Bridges meets with KiwiRail's Chair and Acting Chief Executive at a regular meeting. Mr Bridges says he advised KiwiRail to release the document in full.
9 June - KiwiRail again revises its draft response and tells Mr Bridges' office it now intends to release the document with redactions.
13 June - Chief Ombudsman writes to the Prime Minister, asking for an assurance ministers aren't flouting the OIA. New Zealand First formally requests the Ombudsman investigate.
14 June - KiwiRail releases the document with redactions.
21 June - Chief Ombudsman notifies both KiwiRail and Mr Bridges of his investigation.
27 July - Chief Ombudman releases his findings.