Chief Ombudsman Peter Boshier says the government is entitled to withhold a secret 33-page coalition document from the public.

In his final decision released on Thursday, Mr Boshier said the document - the existence of which was revealed by Deputy Prime Minister Winston Peters - was part of party negotiations and not official information.

"It is quite clear that at this time, (Prime Minister) Jacinda Ardern held the information in her capacity as Labour Party leader," he said.

"Although it was considered during the coalition negotiations, this document did not form part of the final coalition agreement."

The announcement confirms a provisional opinion released by media earlier in the week.

In the final opinion, Mr Boshier said information only became official when it had been used in official ministerial purposes.

"Officials have advised me that the document has not been used by any minister to carry out their official duties," he said.

"It has played no part in policy decisions, and it is not available to ministers as reference material nor does it contain any directives or guidance that ministers apply when making official decisions."

He said he had spoken to Mr Peters' and Ms Ardern's offices during his two-week investigation.

"Mr Peters has also confirmed that the information was created during the course of coalition negotiations between his party, New Zealand First and the Labour Party and does not contain any directives or guidance for ministers."

An Official Information Act request was made by media for the document, which reportedly includes details of "various areas of policy commitment and developments".

Having formed a coalition with Labour in October, Mr Peters promised to release the document.

But Prime Minister Jacinda Ardern said the file was actually just negotiation notes and rejected the OIA application on the basis it was not official information - prompting an appeal to the Ombudsman.

National leader Bill English has argued a fully transparent government wouldn't hide behind the ombudsman.

Ms Ardern maintains the agreed policies reached in coalition talks have been made public, while only undecided potential policy ideas remain undisclosed.