John Ronald Brown, 49, believes his teenage victim is "95 per cent responsible" for the offence he has admitted committing, Professor David Greenberg told the ACT Supreme Court on Monday.

Brown appeared there for a sentence hearing, having pleaded guilty last year to a charge of maintaining a sexual relationship with a child.

The offending involved Brown, who was friends with the victim's parents, molesting the boy at a Canberra party before sexually touching him on an international cruise in late 2019 and early 2020.

His sentence proceedings have dragged on for months, having most recently been adjourned in February because Justice Michael Elkaim wanted to hear expert evidence about Brown from a new source.

The judge asked for this because Professor Douglas Boer, a psychologist, had provided the court with a report that said he thought Brown was not guilty by reason of mental impairment.

The court accordingly heard on Monday from Professor Greenberg, who said he disagreed with Professor Boer's opinion that Brown was suffering from a major depressive disorder at the time of the offending.

He said he did agree with Professor Boer's diagnosis of alcohol use disorder, though he was adamant that such a condition "does not cause a person to abuse children".

Chief Crown prosecutor Anthony Williamson subsequently read out five victim impact statements, which had been written by the boy Brown abused and members of the teenager's family.

The victim said Brown was a person he had once respected, admired and even looked up to.

"He broke my trust," the victim wrote.

"I cannot express how much this has had an impact on me and my family.

"To be told, 'Don't worry about your mum and dad, it's friends' fun, it's our secret', made me sick."

The boy also described the abuse as "disgusting" and "the worst thing that has happened to me".

The victim's father blasted what his "so-called friend" had done to his son as "a violation of a person's innocence", also labelling it "inexcusable, irreversible and unforgivable".

The boy's mother wrote that Brown was "evil" and said she believed the 49-year-old had groomed her entire family in order to commit a crime that was "a parent's WORST NIGHTMARE".

"[Brown] made us believe he was somebody that he is not whilst taking advantage of our son," she said.

"I can honestly say that I have never carried so much hate and feeling of betrayal ever in my life before."

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The victim's sisters also wrote statements, describing how Brown's actions had "broken" and then "haunted" their family.

Brown's barrister, John Purnell SC, conceded the 49-year-old's offending was "clearly disgraceful conduct".

He said Brown would carry "a stigmata" to his grave no matter what sentence was imposed because the offender had "brought disgrace" on himself and his family, while also ruining his friendships with the victim's parents.

But Mr Purnell said Brown, who "gave way to alcohol-induced, disinhibiting behaviour" when committing the offences, had since stopped drinking and engaged in therapy.

"Justice would be served by the imposition of an [intensive correction order]," Mr Purnell told the court.

Mr Williamson countered that anything other than a term of full-time imprisonment would be "manifestly inadequate".

He said the Crown was "very sceptical that [Brown] is genuinely remorseful", adding that the offending involved "an egregious breach of trust".

The prosecutor also told the court Professor Greenberg's evidence showed Brown had exaggerated the extent of his alcohol use, which the offender had effectively used as "a crutch to shift his culpability".

Justice Elkam indicated he would sentence Brown on Wednesday.

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