Former pope John Paul II handled sex abuse claims within the church "poorly" because he simply could not grasp the scale of the problem, a retired senior Bishop in Sydney says.

Geoffrey Robinson, a key player in the church's response to child sexual abuse by members of the clergy between 1994 and 2003, is giving evidence today before the Royal Commission into Institutional Responses to Child Sexual Abuse in Sydney.

Bishop Robinson said he felt the Vatican was far behind in its understanding of the problem worldwide, and that John Paul II — the Pontiff from 1978 to 2005 — showed little leadership on the issue.

"If I am honest, he handled abuse poorly," Bishop Robinson told the inquiry.

Key points: Pope John Paul II handled abuse "poorly", showed poor leadership on issue

Pope John Paul II handled abuse "poorly", showed poor leadership on issue Church, police moved allegedly abusive NSW priest "out of the state"

Church, police moved allegedly abusive NSW priest "out of the state" Bishop Robinson says decision was "outside of the law"

"It didn't fit into his image of church, and he couldn't deal with it."

Bishop Robinson said had John Paul II taken to Vatican Square and spoken in strong terms on the issue, bishops worldwide would have taken notice.

"That would have got worldwide publicity — it would have sent out a message to the whole church," he said.

"Instead, what we got from him was silence. And so bishops were loyal to the silence."

Bishop Robinson also criticised Cardinal George Pell, whose leadership he said was "ineffective" during his time as Australia's most senior Catholic.

"Priests simply forgot the diocese, put their heads down and looked after their parishes, because [Cardinal Pell] had lost the support of his priests," he said.

He said in the 1990s Cardinal Pell, who was at the time the archbishop of Melbourne, decided to create his own protocol called the Melbourne Response while the rest of the church worked on a national response called Towards Healing.

"He destroyed our unity," he said.

Priest 'grabbed boys by the genitals'

The inquiry earlier heard a former NSW priest was shifted out of the priesthood and the state after complaints he was grabbing boys by the genitals and squeezing.

Bishop Robinson said shortly after he became a priest in the 1960s in Croydon, he was told of the actions of the fellow priest, referred to by the inquiry as TS.

"This priest was about three years senior to myself," Bishop Robinson said.

Bishop Geoffrey Robinson is an advocate for a more compassionate approach by the church to abuse victims. ( AAP: Dan Himbrechts )

"He was not the brightest student in the [seminary], but he was probably the strongest.

"At a certain point ... he came across someone doing the wrong thing, and his solution was to revert to his physical strength.

"So he went up the person and grabbed him by the genitals and squeezed. And it worked."

Bishop Robinson said after the priest was moved to a home for delinquent boys, his behaviour became "something of a pattern".

The police became involved, and the decision was made to "get him out of the state and the priesthood", Bishop Robinson said.

"At best it was outside to the law, at worst it was contrary to the law," Bishop Robinson said of that decision.

"They simply decided they knew the facts ... and they acted."

Celibacy not related to abuse by clergy: Robinson

The church's culture, including the issue of celibacy, also came in for examination.

Bishop Robinson said he thought "an awful lot" of priests were only celibate because they had to be, not because they wished to be.

He said that while he did not believe celibacy had anything to do with instances of child abuse among clergy, it was also not "the jewel in the crown" of the church it was made out to be.

But it was also unlikely to be cast aside anytime soon, he said, given the "furore" Pope Francis has already created with his views on remarrying after divorce and homosexuality.

"It's a law — it could be changed tomorrow," he said of celibacy.

"But it's been there for 1,000 years, and popes are very reluctant to change something that has been there for 1,000 years."

Bishop Robinson, who was a bishop for 31 years in the Sydney Archdiocese and a priest for 54 years, has spoken out and written widely urging "profound and enduring change" and a more compassionate approach by the church to victims of abuse by religious leaders.

He is the author of two books on the issue: Confronting Power and Sex in the Catholic Church and For Christ's Sake.