The solicitor-general was only asked to consider whether Morrison had been validly appointed to the role of resources minister in April 2021, and not the other four portfolios he had also taken on.

He found Morrison’s appointment was valid, but was “inconsistent with the conventions and practices that form an essential part of the system of responsible government”.

In response to the advice, Abbott told Sky News on Tuesday: “I’m just not gonna (sic) defend what was done.”

“On the other hand, we’ve got a report from the solicitor-general. It clearly says that there was nothing illegal done, but it also clearly indicates that it is just highly unconventional, highly unorthodox and shouldn’t have happened.”

Prime Minister Anthony Albanese announced an inquiry will be held after the legal advice was released on Tuesday.

When asked if an inquiry was necessary, Abbott said good behaviour was relied upon from the government and its ministers.

“When you don’t get good, sensible behaviour, the best recourse is through the political process,” he said.

“I suppose that’s what we’ve seen happening in Australia.”

Liberal deputy leader Sussan Ley said the coalition acknowledged the advice found the process of appointments could be made more transparent and pledged to work with the government.

“The opposition is ready to proceed along those lines with the government with the proposals to improve those processes,” she said.

“There are issues to be dealt with that the solicitor-general has made us aware of in his report today, and that will happen.”

Morrison said it was important to reflect on some of the decisions and “lessons learned”.

“The solicitor-general has noted a number of these points from his perspective in his advice and I am sure this will help guide any changes in these areas,” he said.

University of Sydney constitutional law professor Anne Twomey said the solicitor-general was given “very little” information, and almost none in relation to what happened within the Department of the Prime Minister and Cabinet.

“They ought to have briefed him with material about what the normal practices are with respect to gazettal (for appointments) and what happened in this case, which resulted in (PMC officials) not telling relevant ministers,” she told Guardian Australia.

Morrison said while many Australians would not agree with, accept or understand the decisions he made, he acted with the best of intentions to “protect Australia in the face of multiple crises”.

He said the legal authorities were valid, there was “no consistent process” for publishing details of ministries, and that ministers had “exercised their portfolio authorities fully” with his confidence and trust “without intervention”.

Morrison maintains his decision to reject the PEP-11 gas drilling project off the NSW coast was the only time he used his extra ministerial powers.

The Greens are seeking a privileges committee inquiry into whether the former prime minister misled parliament.