A depraved former priest was able to carry out a decade-long reign of abuse against three girls and nine boys because he was protected by the Catholic Church, a Sydney judge says.

Loud applause broke out in court on Monday as John Joseph Farrell was led from the dock after being sentenced to 29 years behind bars for his crimes in Moree and Tamworth, in country NSW.

Victims and their loved ones packed into the courtroom at Sydney's Downing Centre District Court to watch the 62-year-old face sentencing for dozens of historical sexual crimes committed against children between 1979 and 1988.

The disgraced ex-priest sat with his eyes closed as Judge Peter Zahra told how he preyed on his victims, grooming the children, cultivating the trust of their parents and exploiting his powerful position as a priest.

"This allowed him to offend whenever and wherever he chose," Judge Zahra said.

"The offender created situations where he was confident he would not be detected even where his sexual abuse was, at times, brazen in the extreme."

Former priest John Joseph Farrell exploited the powerful position he had, a judge said.

The judge said that although there was evidence senior members of the Catholic Church were told of Farrell's crimes many years before he was brought to justice, allegations levelled by complainants "appeared to be frustrated either by the failure of those within the church to pursue any investigation or by dissuading further complaint".

"The offender was thereby protected from the authorities by those within the church who chose to move the offender rather than inform the authorities," Judge Zahra said.

Outside court on Monday, one of Farrell's victims described the church's actions as "despicable".

He is one of a number of victims who are expected to seek compensation in civil proceedings.

"They knew it was happening and they didn't do anything about it," the man said.

"They just pushed it under the rug and tried to forget about it, or paid people to forget.

"It won't be forgotten."

After Farrell forced an altar boy to perform oral sex, the boy told an adult in the presbytery what had happened and was told "they would sort it out."

But when the boy next saw Farrell, the priest raped him behind the altar for what "felt like hours" as he whispered threats in his ear.

"The complainant said that he felt excruciating pain," Judge Zahra said.

"The offender whispered that if the complainant told anyone he would kill him or his family."

Another boy who was repeatedly molested admitted to his mother that Farrell had "touched" him. The mother promptly contacted a monsignor who asked Farrell to leave the parish.

But the priest went on to serve in a number of other parishes and was not suspended from public ministry until 1992.

The court heard that although the complainant went on to contact the nation's most senior Catholic, Cardinal George Pell, in 2002 to ask why the church had not responded to his family's allegations, Farrell was not laicised until 2005.

Around the time of the initial disclosure, the victim's mother also sought advice from a nun who told her "the church had a large amount of money to fight such allegations".

Farrell was given an 18-year minimum sentence and, with time already spent in custody, his term will expire in 2044.

(With AAP)