A friend of former ombudsman Allan Asher has been locked in a battle with the department after it refused to release the briefing under freedom of information law.

The documents relate to events in 2011, which saw Mr Asher script questions for Greens senator Sarah Hanson-Young ahead of a Senate estimates hearing.

The questions related to the then Labor government's underfunding of the Commonwealth Ombudsman's office, and Mr Asher later said he had used an "unorthodox approach" to get his concerns voiced in Parliament and public.

Mr Asher's office had just been handed responsibility for looking at immigration detention on Christmas Island, without any additional funding to help it cope.

The ombudsman – whose role is to ensure government administration is kept fair, just and accountable – is supposed to remain impartial and independent, and above party politics.

Revelations about the briefing of the Greens senator prompted criticism and concern from the Liberals and Labor, and Mr Asher resigned in October 2011.

Then Greens leader Bob Brown described the ombudsman's treatment as a "political assassination of a very decent Australian, working in the public interest".

Three weeks before Mr Asher's resignation, two of the Department of Prime Minister and Cabinet's most senior bureaucrats – secretary Ian Watt and first assistant secretary Philippa Lynch – helped prepare a briefing on the controversy for then prime minister Julia Gillard.

The briefing was prepared urgently and contained content that, at the time, was considered "highly sensitive".

A friend of Mr Asher, John Wood, later submitted a freedom of information request for documents, including the ministerial briefing.

The department released all the documents bar the briefing, and Mr Wood took the case to the Freedom of Information Commissioner, who decided the briefing should be released, except for some passages, which were subject to legal professional privilege.

The department, however, appealed to the Administrative Appeals Tribunal, arguing more of the briefing should be kept secret under provisions restricting the release of advice, recommendations, or opinions given to a minister as part of a deliberative process.

Prime Minister and Cabinet deputy secretary Elizabeth Kelly argued in an affidavit that the disclosure of such briefings would hamper the public service's ability to provide frank and fearless advice.

"Public servants will understand they have a duty to give the best advice they possibly can in the circumstances," she said.

"But public servants will also be concerned about the possible consequences for the incumbent in that role, and for the office itself, in disclosing sensitive advice about the discharge of functions in this offices.

"The possibility of the public disclosure of advice options canvassed could prejudice or pre-empt a proper consideration of the issue by relevant decision-makers is likely to encourage the advice giver to be more circumspect in the giving of that advice."

Tribunal deputy president Stephanie Forgie agreed public servants needed to be able to give completely frank advice to ministers.

"Having acknowledged that, however, I do not accept that advice given on those occasions need always be kept confidential and out of the public eye," she said.

"In this case, four years have passed since Mr Asher spoke with Senator Hanson-Young regarding matters relating to his office. Mr Asher has resigned as ombudsman. Ms Gillard is no longer the prime minister. Mr Watt is no longer secretary of PM&C."

"It is difficult to see how disclosure of such advice some four years after the circumstances in which it was given will inhibit public servants in giving such comprehensive advice in future."

Deputy president Forgie found that there was a public interest in disclosure of the information, and gave Mr Asher and Mr Wood access to all parts of the briefing, except for those that fell under legal professional privilege.

The decision may be subject to an appeal and it is understood the briefing has not yet been released to the parties.