The vow of celibacy is not the driver of sexual abuse by clergy, offending which can leave shattered children believing that "even God's bad", an expert says.

Abuse occurs in every faith but it seems to be more marked in Catholic institutions, psychiatrist and University of NSW Associate Professor Carolyn Quadrio told the child sex abuse royal commission.

She said the Catholic Church rule requiring celibacy of its clergy is important, but not the driving cause.

"I don't think the celibacy drives child abuse," she told commission's hearing into Catholic institutions in Ballarat on Monday.

Prof Quadrio said men who already had an orientation to be attracted exclusively to children as sex objects would feel more comfortable in the priesthood.

"The celibacy vow is not going to bother you if you're not interested in having sex with other adults, so obviously that will be a more comfortable environment," she said.

"They have access, they have authority and they have the cover of a very respected profession."

Prof Quadrio said children who have been abused can feel guilt, shame and self-doubt as they don't understand that it's not their fault.

"They feel dirty, defiled, damaged. They blame themselves: `I'm a bad child, it wouldn't have happened to me if I wasn't bad'."

She said abuse in a religious context could be particularly damaging as children view a member of the clergy as someone close to God and could grow up feeling "everybody's bad and even God's bad".

"The sense of betrayal is particularly shattering because it's kind of like not just one bad person, but it feels like, well, maybe God's bad," she told the commission.

"The loss of faith and shattering of the belief is really very damaging to a child."

She said the child could get a negative response from their strongly religious family or community if they told of the abuse, which could compound the damage.

Prof Quadrio said a proportion of abused children will go on to become offenders themselves, although it was not the majority.

She said about 30 per cent of girls and 20 per cent of boys in institutional care were abused.

Prof Quadrio said the institutions where children were abused should not be providing the treatment.

"Maybe the institutions can pay the bill but that's all. They shouldn't be providing the treatment."

David Ridsdale, a nephew and victim of pedophile priest Gerald Francis Ridsdale, said there had generally been an outpouring of support during the Ballarat hearing.

But he said a survivor was harassed in a car park and told "he was scum and we were all scum for speaking out".