David Thodey, who has recently completed the final report of the review, made the comments during a wide-ranging discussion of falling levels of trust in the public sector at Australia National University on Monday night.

In a room filled with current and former senior executives, business figures and academics, Mr Thodey said he did not believe the APS was failing to give frank and fearless advice to ministers.

But he said there had been a long-term hollowing out of expertise in the public service, particularly in service delivery, which had been largely subcontracted to consultants, leaving public servants distributing funds.

He said he was impressed with the depth of talent in the public service, despite the need for more training.

While Mr Thodey could not go into detail on what his review had recommended, he said he took faith from the fact Prime Minister Scott Morrison had taken it upon himself to hold the public service portfolio.

Mr Thodey said given former prime minister Malcolm Turnbull had endorsed the review Department of Prime Minister and Cabinet secretary Martin Parkinson had proposed, he had not spent the past year working on it for it to be shelved.

He said he wanted it to lead to meaningful change, and had found a major need for today's promising young public servants to be given the support and training they needed to lead the public service in 20 years' time.

During the forum, former foreign minister Gareth Evans said he believed the public service had become like the lover who had become the husband, once the nerve had been removed - referring to theories the APS had long lost its courage in ministerial advice.

But Mr Thodey said he believed that generally that was not the case, though there was a lack of expertise, or where there was expertise, the political class was not listening to the advice.

Home Affairs secretary Michael Pezzullo also said he had not seen any erosion of courage in the public service advice to ministers.

Mr Pezzullo also said the landscape had changed much with social media and its role in presenting two different realities to people, and while the public service could learn from history, things also changed over time.

Mr Thodey also said he was surprised at the lack of understanding in the public service ranks about the private sector, and there remained a need for more collaboration between the APS, the private sector and academia.

He was also concerned the public service needed to rebuild trust in the general community in both the policy expertise that still existed, as well as its capacity to deliver services, despite the need to improve capability.

While there remains concern about senior public servants being sacked along allegiance lines, Mr Thodey said he believed that was less of an issue than the political masters actually acting on APS advice.