The conviction of a Christian Brother who confessed to abusing "unknown" children prevented other men from coming forward to report he abused them, a royal commission has heard.

The Royal Commission into Institutional Responses to Child Sexual Abuse has also heard bringing sexual assault allegations 40 years after they happened was considered an extraordinary thing in the 1990s.

Brother Dick was convicted and sentenced to three and a half years jail in 1994 after he confessed to 10 counts of unlawfully and indecently assaulting five "unknown" boys under the age of 14.

The commission heard double jeopardy laws prevented other men who were abused as children by the Christian Brother from coming forward because it was not known if they were among the five he had been convicted of abusing.

In response to a question on Monday, Bruno Fiannaca, deputy director of public prosecutions in Western Australia confirmed this would likely be the case.

"If the actual sexual act complained of was the same and the circumstances described were the same within the same time frame, then I think that's correct," he said.

"It would be impossible then to know whether one would be charging for an offence for which he has already been convicted and the law of double jeopardy I think would be well known to the general community."

The Perth inquiry is investigating the responses of the Christian Brothers and relevant West Australian state authorities to the abuse allegations at the residences at Bindoon, Castledare, Clontarf and Tardun between 1947 and 1968.

The commission also heard the department found allegations emerging some 40 years after they happened to be extraordinary.

"You would understand that in many child sexual complaints to the police, they are made many, many years after the events occur," counsel assisting Gail Furness SC asked Mr Fiannaca.

"Yes, but even so, I must say 40 years was quite extraordinary at the time," he replied.

The commission is next expected to hear from the former provincial leader for the Christian Brothers in Western Australia, Brother Anthony Shanahan, as well as the current deputy provincial leader for Oceania, Brother Julian Mcdonald.