An Australian Catholic priest was sentenced yesterday to almost 20 years in jail for child sex attacks which the judge said were sadistic and went unpunished for many years.

John Sidney Denham, 67, was sentenced to 19 years and 10 months after pleading guilty to a range of charges, including multiple counts of indecent assault against boys aged five to 16.

Judge Helen Syme told a Sydney court that many of Fr Denham's 25 victims were left terrified by the attacks they endured at schools in Sydney and elsewhere in New South Wales between 1968 and 1986.

"The indecent assaults involved multiple children, often significant planning, were frequently sadistic and overall persistent, objectively serious, criminal courses of conduct," Judge Syme said, according to Australian news agency AAP.

"The offender's actions contributed to a culture of fear and depravity, especially at the school, which allowed these disturbing offences to occur and then remain unpunished for years."

The public gallery applauded the sentence, of which Fr Denham must serve a minimum of 13 years and 10 months.

Fr Denham had said in court on Thursday that he was still trying to work out why he had abused the boys.

"All I can say is, I am so sorry," he said. "I see myself as a mere scumbag paedophile who took advantage of a situation and used my power to abuse young people."

But Judge Syme said she was not convinced that the priest, who has been in custody since 2008, was genuinely remorseful.

Outside court, the mother of one of the victims said Fr Denham's actions had "ruined families" and should have been acted on by the Catholic Church sooner.

"It's disgusting, the hierarchy in the Catholic Church. I can't believe it," she told reporters. "I can't believe I sent my children to a school like that."