As a former doctor, Professor Faunce has had to call in public guardians to make decisions as fundamental as turning off life support and donating organs. He said such highly sensitive decisions must be made by people with the skills and training of public guardians and not by public trustees.

"You never want to get into a situation where life and death decisions are hanging because of financial considerations or that there was any perception that financial considerations would be one of the dominant factors," he said.

Professor Faunce was speaking ahead of a decision expected in the ACT Parliament on Tuesday to merge the guardian and trustee offices. The government is determined to push ahead with the merger in the face of trenchant opposition from the guardian staff and from many human rights organisations.

Such is the unhappiness within the guardians' office that at one point in November all three senior guardians were on leave. One subsequently left, and another was on open-ended leave in December.

A former head of the public guardians, Heather McGregor, has described the merger as "a travesty" and "backward step for our most vulnerable citizens".

Attorney-General Simon Corbell has dismissed concerns, saying "financial imperatives will not trump other considerations".

He has already appointed public trustee Andrew Taylor as head of both organisations.

However, Professor Faunce said the jobs of guardian and trustee were fundamentally different, with guardians having specialised communication skills and making decisions for vulnerable people unable to manage thier own lives.

"There are a whole stack of complex decisions that guardianship people are skilled in dealing with, it's a very contentious area and you really want to feel their particular expertise and training is being valued," he said.

"If you have one organisation, how are you going to maintain appropriate Chinese walls within the organisation? Because you simply can't have people who have got training in being a public trustee working as guardians for people."

Professor Faunce also pointed to the corruption allegations that hit the public trustee's office in 2014, with two former staff and two contractors accused of stealing $1.65 million from clients.

Having a separate guardians office helped provide a check on corruption.

"Putting these two agencies together makes it more likely that there's going to be problems because once something's in-house with just one agency it's much easier to keep the problem in-house," he said.

Professor Faunce has a joint appointment in the ANU College of Law and its Medical School, after 20 years as an intensive care doctor. He recalls calling in the public guardian to make decisions about life support and organ donation.

"Their role is to step into the shoes of the patient and try and evaluate what their best interests are in that situation," he said. "So it's a very skilled role and it's not something that you could easily imagine someone from the public trustee doing."

While Mr Corbell has rejected suggestions of a conflict of interest between the two roles, Professor Faunce predicted that the drive for efficiency would eventually lead to the appointment of just one person to look after a client's finances and make their life decisions.

Public trustee Mr Taylor has already pointed to the potential to save money, saying more than 130 people have both a guardian and a financial manager, and having one agency do both jobs would "generate efficiencies". He has also raised the possibility of charging people for the guardian services, but Mr Corbell has ruled out the idea.

The Liberals are yet to decide a position, but irrespective of their position, the government can make the change with the support of the Greens' Shane Rattenbury.