Professor Paul Mullen's resume reads like a who's who of the nation's most high-profile criminals.

He interviewed Martin Bryant, the man who shot 35 people dead in the Port Arthur massacre in 1996, and Hoddle Street mass murderer Julian Knight, who he more recently assessed as being unlikely to repeat the random shooting spree that killed seven in Melbourne in 1987. He worked on the Snowtown murders in the 1990s, also known as the bodies-in-barrels case, which has been described as Australia's worst serial killings.

Professor Mullen has also worked as an expert assessor for the US Army defence team acting for former Guantanamo Bay detainee David Hicks, and assessed Bali nine pair Andrew Chan and Myuran Sukumaran years before their execution.

The court appointed him as its expert in the Chelmsford Hospital medical negligence case, the largest of its kind to date, which sued for the devastating use of electroconvulsive and deep-sleep therapy on patients with mental illness.

Since 1979, Professor Mullen has given expert evidence in criminal courts, both for prosecution and defence, and has worked extensively in the ACT jurisdiction.

The research of the Victorian-based forensic mental health expert and his colleagues is considered among the best in the world.

He was formerly the clinical director of Victorian Institute of Forensic Health, and has spent more than a decade as a professor of forensic psychiatry at Monash University.

Professor Mullen's most recent work in the ACT was on the case of Luigi Costa, 72, who was found guilty of the brutal and bizarre killing of his neighbour in the exclusive Canberra street of Mugga Way, Red Hill. He assessed Costa as being seriously compromised by dementia and alcohol at the time of the murder.

Professor Mullen had been due to give evidence in the upcoming trial of Christopher Navin, 29, who is accused of stabbing an old school friend to death in a Lyneham home.

But Mr Navin's lawyer, Peter Woodhouse, told the ACT Supreme Court last week that Professor Mullen had written to him, advising he had resigned his medical registrations and college positions.

The health of the forensic psychiatrist, the court heard, had deteriorated to such a point that he was no longer able to give evidence or continue practising.

When approached for comment, Professor Mullen replied: "Thank-you for your interest. I am over 70 and my career as a clinical psychiatrist is now over. I have nothing else to add."

The sudden resignation forced the delay of Mr Navin's trial by three to four months.

Mr Woodhouse said Professor Mullen's evidence was crucial to determining whether Navin was mentally impaired, which would be a central issue at trial.

The researcher had interviewed Mr Navin three times, the court heard, and had worked on the case since the arrest.

"That leaves us in a significantly prejudiced position, your honour," Mr Woodhouse said.

Justice John Burns agreed to vacate the trial date last week, and the trial is now likely to take place in November and December.

Professor Mullen is also involved in the review of Victoria's system for managing serious sexual offenders, ordered by the state government in May, and due for completion in October.

It is unknown whether his resignation will affect that work.