Former Catholic school teacher Darcy John O'Sullivan, who indecently assaulted at least a dozen children in New South Wales in the 1970s and 80s will spend at least three years behind bars.

Also known as Brother Dominic, O'Sullivan was a teacher at Marist Brothers in Hamilton, Newcastle, and later a principal at St Mary's High School in Casino.

O'Sullivan, 78, pleaded guilty earlier this year to 22 counts of indecently assaulting 12 boys at both schools over a 13-year period.

"[He] became increasingly confident that these offences would never come to light," District Court Judge Kate Traill said in her sentencing remarks.

The victims, she said, were left feeling "humiliated and degraded by the offender and they would have found it difficult to report [the assaults]."

Judge Traill pointed to a psychological assessment where O'Sullivan said "[He] saw the behaviour as an outlet of physical affection rather than for sexual gratification."

She also expressed frustration at having to sentence O'Sullivan in line with laws that were in place at the time of the offences.

Judge Traill said the same offences now attracted a maximum sentence of up to 10 years' jail as there was now a better understanding of their impact upon victims.

Court told of 'brazen' assaults

Earlier in the trial, the court heard O'Sullivan "made a lifestyle" of preying on young students.

During the trial, evidence was given that some students at the school were sent to his office for punishment, where he would ask them to sit on his knee while he touched them inappropriately.

Another victim told police he reported the abuse to his mother who said: "You shouldn't have been sent to his office in the first place, it's your fault."

Crown prosecutor Sean Hughes said the assaults were "brazen", "witnessed by other students" and that "during the time of his offending ... the trust in the clergy was most substantial".

"He had a commitment to enrich the lives of these boys, he did quite the opposite - he's caused substantial damage," Mr Hughes said of O'Sullivan's actions.

O'Sullivan had been out on bail during the trial, but it was revoked last month when Judge Traill indicated a full-time custodial sentence would be imposed.

At the time, several of the men assaulted when they were young boys watched as he was taken into custody by corrective services officials and said they were "delighted" at the outcome.

O'Sullivan will be eligible for parole in 2019.

Earlier this month, the Royal Commission into Institutional Responses to Child Sexual Abuse examined the response of the Marist Brothers to allegations of sexual abuse made against several staff over decades.

One victim identified as CNQ described being assaulted by O'Sullivan after being sent to him to receive the cane in 1977.

He also told of another incident: "It hurt a lot and I screamed."

"Afterwards I was dazed and was wandering around the lunch area."

O'Sullivan will be eligible for parole in 2019.