Justice Michael Elkaim sentenced John Ronald Brown on Wednesday to two years, six months and 14 days in jail for an offence of maintaining a sexual relationship with a child.

The ACT Supreme Court judge ordered Brown, who had previously pleaded guilty, to serve a minimum of 17 months without parole.

"All older men contemplating molesting children must know that, when they are caught, they will be punished appropriately," Justice Elkaim said as he rejected the 49-year-old offender's request for an intensive correction order.

Brown's supporters and members of the victim's family watched on from opposite sides of the courtroom as Justice Elkaim described the "offensive relationship" that spanned about six weeks in late 2019 and early 2020.

The judge said the crime had involved Brown taking advantage of a 15-year-old boy to carry out "four distinct sexual acts" at the child's Canberra home and on an international cruise the pair's families embarked upon together.

Brown and the boy exchanged text messages after one of the earlier incidents, which happened at a party where the child lived, agreeing to "never repeat it".

The judge said Brown had perhaps made this agreement because he was "consumed with guilt at his appalling conduct", or maybe because he was afraid of the possible consequences.

"If the offender had been genuinely remorseful, the illegal conduct would not have continued," Justice Elkaim said. "But it did."

The judge described how the offending had continued on the cruise ship and only stopped when the victim's father witnessed the last sexual act being performed on his son.

The father immediately told the victim's mother, who confronted Brown about it on the ship.

"[Brown] endeavoured to blame the victim," Justice Elkaim said, also noting the offender had, since being reported to police upon the ship's return to Australia, demonstrated "a persistent belief ... that he is not entirely to blame".

The judge also referred during sentencing to five victim impact statements, which were written by the boy Brown abused and members of the teenager's family.

He said these statements, which were read to the court on Monday, "overwhelmingly demonstrate that in crimes of this sort, there is very rarely a single victim".

In this case, the court heard the victim's whole family was leaving Canberra to escape "painful memories" of the "evil" Brown and the crime that had badly damaged both the families involved.

MORE COURT AND CRIME NEWS:

Part of Brown's argument for an intensive correction order was based on the many medical issues, including morbid obesity, sleep apnoea and "fatty liver", which would make life behind bars harder for him than the average person.

The court heard a psychologist had also suggested at one point that Brown might have been mentally impaired to the extent he did not understand the wrongfulness of his offending at the time in question, but Justice Elkaim found this was not the case.

While the judge accepted Brown had "impaired mental functioning" as a result of general anxiety, depression and excessive drinking, which involved having up to 30 beers a night, "there is no suggestion that any of the sexual acts occurred while the offender was so intoxicated he did not know what he was doing".

Justice Elkaim ultimately found Brown's crime was "simply too serious" for anything other than a term of full-time imprisonment.

Brown will become eligible for release from prison in December 2023.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: