Garry Leslie Marsh, 73, was sentenced to four years in prison on Wednesday after he pleaded guilty to five counts of indecent assault against his former student in the 1970s.

He was given a two-year non-parole period and will be eligible for release in September next year.

The court heard the abuse occurred on several occasions and included Marsh applying Vaseline to his genitals and using the 10-year-old boy's hand to pleasure himself and on another occasion digitally penetrating the boy.

Justice Michael Elkaim said "it is difficult to imagine more depraved behaviour upon a young boy".

The victim read out his statement telling the court of the trauma and lasting effects he had suffered due to the abuse.

He said he had no fond memories of school and Marsh's actions had caused him to become uninterested in his schooling and he had not pursued any further study after school.

He said he had been diagnosed with post traumatic stress disorder, had turned to drugs to cope and in more recent years had become reliant on alcohol to help him sleep.

He had been unable to share what had happened to him at school with his family but said without their support he might have succumbed to taking the path some other survivors had followed.

"I will never forget you and the evil things you did," the victim said to Marsh.

"I will never forgive you.

"I do not have the chance to reduce my sentence, mine is for life."

Marsh's defence barrister, Greg Walsh, described the man's statement as "an extremely eloquent and honest account of the impact the offending had on him".

He described Marsh's actions as an "egregious breach of trust" and he said all children had the right to be safe from abuse.

"At long last, [Marsh] conveys to his victim vindication," Mr Walsh said.

However, Mr Walsh also asked Justice Elkaim to spare his client from another term of imprisonment.

He told the court how Marsh had been previously incarcerated and was recognised as a child sex offender. Mr Walsh said Marsh had been assaulted on his second day in prison and had had his food thrown onto the floor of his cell.

At his age and having required recent heart surgery, plus being his 97-year-old mother's primary carer, Mr Walsh said prison would be a significantly more taxing punishment on his client.

"The sad thing is he is to be sentenced as he is today and not as the man he was all those years ago," Mr Walsh said.

However, Justice Elkaim determined the appropriate sentence was to send Marsh back to prison.

Marsh had previously been convicted of offences towards the same victim but had had the conviction set aside on appeal.

He eventually pleaded guilty once alternate charges were brought against him by prosecutors.

Justice Elkaim said the community needed this type of conduct to be utterly condemned by the court.

"Anyone who contemplates this kind of activity must understand they will eventually be punished even if it is many years after the crimes were committed," Justice Elkaim said.

Marsh was taken from the court by ACT Corrections officers to the Alexander Maconochie Centre.