Scott Morrison is calling for a stack of secret cabinet documents to be made public before he gives evidence at the robodebt royal commission.
Lawyers for the former prime minister argue his reputation could be damaged if the confidential papers are kept under wraps.
Mr Morrison was social services minister when the scheme was established in 2015 and prime minister when it was wound back in 2019 after its income-averaging debt calculation method was ruled to be unlawful.
Mr Morrison's lawyer, James Renwick, argued there would be "considerable difficulties" in the former PM giving evidence without being able to refer to documents redacted under public interest immunity claims.
"If he says I did 'X' because of 'document Y' and you do not have document Y ... you will never be able to come to a finding based on all of the evidence ... you have failed to inquire in a manner where a document is readily available to you," he said.
Commissioner Catherine Holmes said she was "puzzled" by the application.
She accepted three specific examples where redacted documents may need to be amended, but fell short of extrapolating that across the wider range of cabinet documents.
"The argument on these three items looks pretty good but I don't see that it follows across the board," she said.
"I just don't understand why these documents ... get anybody any further."
Dr Renwick responded: "Because, commissioner, his reputation is on the line".
Ms Holmes clarified the general presumption was a shared responsibility for cabinet decision-making rather than an individual minister.
A private commission hearing on Thursday will address the issue further.
Mr Morrison is scheduled to give evidence the following Wednesday.
It came on another bombshell day of evidence where it was found the scheme continued for months after the Human Services department received legal advice it was probably unlawful.
The royal commission heard it avoided a Federal Court hearing that would have ruled on its legality, dropping a $4000 debt in 2019 after it was challenged in court.
Legal advice from an Australian government solicitor the department received in March said they had a "very good" chance of losing the case.
The scheme wasn't wound back until November 2019, eight months after the legal advice was received.
Former DHS general manager Craig Storen said he believed the advice was only relevant to the case involving Ms Masterton.
An incredulous Mr Scott said that was clearly not the case.
"That advice implicitly stated the use of ... income averaging was not a lawful basis to calculate a debt," he said.
"It was commenting on the provisions of the social security legislation that didn't just apply in the (Madeleine) Masterton case, it applied to social security recipients more generally."
Ms Holmes agreed, asking: "You thought there was no statutory basis regarding Ms Masterton, but for everyone else it was fine?"
It is investigating how the scheme, which operated between 2015 and 2020, went ahead despite departments knowing the debt calculation method was unlawful.
