Public service commissioner John Lloyd has refused to rule out whether he is under investigation in a two-hour barrage of questioning from Labor senators following a Prime Minister's Department decision not to release emails relating to him and a right-wing think tank.
The Department of Prime Minister and Cabinet has withheld emails relating to Mr Lloyd and the Institute of Public Affairs following a freedom of information request, saying they could prejudice an investigation into a possible breach of the law.
Mr Lloyd has previously rejected suggestions he gave special access and research to the IPA after Labor senators last year raised an email he sent to a member of the group with an attachment showing what he described as "generous" provisions in public service enterprise agreements.
He refused to comment on Monday despite repeated questions from Labor at a Senate estimates hearing over his knowledge of an investigation referred to in the PM&C; freedom of information decision.
Labor senator Penny Wong told Mr Lloyd, a member of the IPA, under Senate rules he could not simply refuse to answer the question and that his response was not a claim to public interest immunity.
"I know it's embarrassing for you, but you don't actually have the option of simply saying 'I'm not going to talk about it'," she said.
"It is not appropriate for you to simply say 'I'm not going to comment' when asked that question."
Mr Lloyd first said he would not comment because his role required him not to disclose the identity of people subject to an investigation.
"I respect that very religiously when I'm required to investigate a complaint against a chief executive officer. I respect it very strongly in that I do not disclose the identity of the complainant or respondent of the complaint," he said.
Neither Mr Lloyd nor any Coalition senator had appeared to make a public interest immunity claim before he refused to respond.
Following repeated questioning, Labor senators held a private meeting and Mr Lloyd said he would take on notice the basis on which he could make a public interest immunity claim not to answer questions about an investigation.
He did not give a deadline before which he would respond to that question.
Liberal senator and Senate president Scott Ryan, sitting with Mr Lloyd earlier during questioning, had to seek further advice about a public interest immunity claim and returned to say he would not file one. However he believed the APS commissioner could make a claim.
A freedom of information request in January sought emails held by PM&C; secretary Martin Parkinson mentioning Mr Lloyd and the IPA, and dated from October 23, after senators referred to the email in a Senate estimates hearing.
The department responded to the request last month by refusing to release two emails in Dr Parkinson's inbox, dated December 20 and December 22.
"I am satisfied that disclosure of the documents could reasonably be expected to prejudice the conduct of an investigation of a breach, or possible breach, of the law in a particular instance," assistant secretary Peter Rush wrote.
Releasing the documents could also "reasonably be expected to prejudice the impartial adjudication of a particular case", Mr Rush said.
One document is 30 pages long, and another is five pages.
The department and the Australian Public Service Commission have refused to answer repeated questions from Fairfax Media asking who is under investigation, who is conducting the probe, and the matters being investigated.
Other APSC officials told senators they had seen the FOI decision but were not aware of an investigation, and Liberal senator Michaelia Cash said she was also unaware.
The public service commission has previously said Mr Lloyd's email correspondence with the IPA upheld the APS value of impartiality.
Before the Abbott government appointed him APS commissioner in 2014, Mr Lloyd was the director of the IPA's work reform and productivity unit.