Justin Gleeson, SC, said the community should demand public officials with sweeping powers brought "high standards of conscience to bear" in decisions affecting a person's freedom or security that would likely not be scrutinised by the court system.

Mr Gleeson resigned his post as solicitor-general last November after a bitter public stoush with Attorney-General George Brandis, sparked by his decision to prevent ministers seeking expert legal advice without his written approval.

He did not comment on the drawn-out row, but spoke more broadly on Law, Morality and the Public Trust as he delivered the annual Blackburn Lecture at the Australian National University to mark Law Week.

Mr Gleeson, who has since returned to private practice, said recent high-profile court decisions involving public officials, such as former senator Bob Day and former NSW minister Eddie Obeid, showed the country's legal system upheld the public trust.

But he said the community had more work to do as there remained important cases where citizens "can and should demand that those in power, and those advising those in power, will bring high standards of conscience to bear".

"Particularly in cases where the citizen or subject may be most at risk of exposure to excessive use of state power but least able to resort to the courts or tribunals of Australia for relief," he said.

Mr Gleeson said broad powers handed to government ministers meant there would be cases where "morality rather than law might be the guide" in critical decisions where executive power was exercised without parliamentary accountability.

Such cases could relate to a minister authorising the rendition and interrogation of an overseas suspect, or giving permission for electronic communications to be intercepted without their knowledge, in decisions that would not likely face scrutiny in court.

In those situations, it would be only the "well-formed conscience" of the minister and their advisers to make decisions that upheld the public trust, he said.

Mr Gleeson also said the angst "legitimately" felt by thousands of welfare recipients targeted by Centrelink's robo-debt system could be seen as "a failure of the law" to be accessible, understandable, capable of being complied with, and challenged in a court or tribunal.

"Some amongst the most vulnerable in our society received threatening legal documents in circumstances where it was difficult for them to understand the demands being made, whether the demands were justified in law and where it was difficult to obtain accurate information and advice on those questions from the government, and their access to community legal centres or other advice was limited," he said.