While Cardinal George Pell, leader of the Archdiocese of Sydney, on Tuesday continued to describe the instances of child sex abuse as "historic", psychologist Andrew Fuller said he was treating children who had suffered abuse at Catholic schools recently.

"Kids from all types of schools suffer; there is all sorts of abuse," Mr Fuller said.

"It is pretty horrendous. It is certainly something that has occurred recently. A royal commission is something I would support, but I do think it is almost a bottomless pit," he said.

Mr Fuller said he treated schoolchildren and adults who were abused in Catholic schools and non-Catholic schools. Recent Catholic perpetrators could be a clergyman or a teacher, he said.

"There is a mixture of both – sometimes it is teacher and sometimes clergy – there is a very strong protective element for these adults."

Mr Fuller said he hoped a royal commission would stop a culture of protection of adults accused of child sex abuse.

He said where children were abused there was a need for pastoral care and counselling.

In talking to their children, parents should discuss the information that would come from the royal commission without too much specific detail, he said. They should allay children's concerns but use the time as an opportunity to talk about trust.

"I do work with a lot of Catholic schools and there are a lot of terrific schools. There are many, many schools where children are very safe, [and in a] very loving and caring environment," Mr Fuller said.

"We do have to teach our kids to look out for the signs that someone is not worthy of that trust. You need to do it in a measured, matter of fact way.

"Parents should be aware of signs of trauma. With any kind of trauma, some kids are going to latch onto it more than others and parents need to look for changes in their children that might show they are being affected by what they are hearing and seeing," he said.

Sleep disturbance, hyperactivity or lethargy were some signs children were affected by trauma. Parents should talk about children's thoughts and feelings without asking them leading questions.

Mr Fuller said a royal commission would be difficult for Catholics generally because it could lead to a crisis of faith.

"I imagine many Catholics might lose faith over what they hear. It is sad, but it is not a reason for people to lose what they believe in. For people who are religious, [religion] is often part of who they are."