Former prime minister Scott Morrison has been warned to stop leaking national security information to journalists.
Mr Morrison revealed he had secretly taken on five ministries to journalists Geoff Chambers and Simon Benson for their book, Plagued.
He also provided sensitive cabinet and national security information.
Attorney-General and Cabinet Secretary Mark Dreyfus wrote to Mr Morrison on Monday warning him to cease leaking.
"I am writing to you to express concern at the apparent extensive disclosures of cabinet information," Mr Dreyfus wrote.
"Several disclosures appear to have been made in contravention of the expectation of discretion regarding sensitive cabinet discussions."
Discussions included the disclosure of cabinet being briefed about the national security implications of COVID-19.
References to secret intelligence briefings were also singled out.
"Disclosures of cabinet discussions and deliberations undermine cabinet confidentiality and solidarity," Mr Dreyfus wrote.
"I trust there will be no further disclosures from your period in government that undermine national security and the integrity of the cabinet process."
Meanwhile, a Senate estimates committee heard the attorney-general at the time of Mr Morrison taking on multiple portfolios did not receive legal advice from his department about the matter.
The current head of the attorney-general's department told parliamentarians she was not aware of then-attorney-general Christian Porter providing any advice to Mr Morrison either.
"Based on inquiries I've taken, the department did not provide advice to the then-attorney general on that issue," Katherine Jones told the committee on Monday.
Ms Jones was not the secretary at the time.
An independent inquiry into the matter is being undertaken by former High Court Justice Virginia Bell.
Mr Morrison secretly appointed himself to five portfolios - health, finance, treasury and home affairs and energy and resources - between March 2020 and May 2021.
The solicitor-general's advice on the legal implications of Mr Morrison's decision to secretly swear himself into the resources ministry found his actions were "inconsistent" with responsible government.
Mr Morrison maintains the swearing-ins were legally sound and were necessary to deal with extraordinary circumstances.
But he said the only time he used his extra powers was in the case of a resources project.
Mr Morrison has been contacted for comment.