An ACT court has found a former Marist College Canberra teacher unfit to be tried on more than a dozen sexual abuse offences allegedly committed against boys in the 1980s.
The court's decision means John William Chute, known as Brother Kostka, 85, will instead face a special hearing in the ACT Supreme Court and, even if found guilty, will likely avoid conviction.
Chute was charged with 13 sex offences allegedly committed against four children between 1979 and 1986.
The alleged offences include one act of buggery, 10 indecent assaults on a male and two acts of indecency on young people.
He was among four men police charged in February last year as part of an ongoing investigation, Operation Attest, into historical child sex abuse, mostly on boys younger than 16, in ACT schools.
Chute was committed to stand trial but had yet to enter pleas to any of the charges.
The court previously heard Chute suffered from Parkinson's disease and dementia, and had been found unfit to plead in the NSW District Court.
Chute returned to court on Monday for a hearing to determine whether he was fit to be tried.
His defence lawyer, Greg Walsh, told the court four medical experts, including two experts enlisted by the Crown, who prepared reports agreed Chute was not fit to plead to criminal charges.
"It's a fairly overwhelming situation," Mr Walsh said.
The prosecution conceded Chute was unfit to plead.
In her decision, Chief Justice Helen Murrell cited one psychiatrist who said Chute suffered Parkinson's disease which would likely progress to dementia.
His mental state would likely mean he would likely be unable to follow trial proceedings or provide reliable instructions to his lawyers.
Another psychiatrist found evidence of dementia and said Chute would also probably have difficulty recalling details and retaining information at trial.
The condition was permanent and not likely to improve in the next 12 months, the Crown expert found.
Chief Justice Murrell said the reports were unanimous in finding Chute had advancing dementia.
"The expert evidence clearly establishes he is unfit to be tried and will not become fit to be tried within the next 12 months," she said.
Chute will now face a special hearing, similar to a trial, under the ACT's mental health laws.
The judge's finding means if the jury in the hearing finds Chute guilty of the alleged offences, that verdict isn't grounds for a conviction to be recorded.
The matter could then be referred to the ACT Civil and Administrative Tribunal for review, or for the tribunal to make a mental health order.
Marist College's response to past allegations of abuse by Brother Kostka was a significant focus of the Royal Commission into Institutional Responses to Child Sexual Abuse hearings in Canberra.
The school was found to have shuffled him between schools upon learning of complaints that he inappropriately touched children.
Chute's case will return to court next month. A date is yet to be fixed for the hearing.