The 65 year old Sydney man, whose name has not been released, was charged in May with 33 child sex offences as a result of a police investigation into allegations of abuse St Stanislaus' College in the city of Bathurst, 200 kilometers west of Sydney, police Superintendent Michael Goodwin told Australian Broadcasting Corp radio.
"We've established five victims from our initial inquiries and subsequent to that, we've had a further eight people come forward," Goodwin said. "We're looking at 13 victims who are cooperating with the police at this stage."
One of the alleged victims, who is now 35 years old, told Sydney's The Daily Telegraph newspaper that boys at the school were subjected to horrendous sexual abuse during night prayer sessions and were sometimes forced to abuse each other in orgies.
Goodwin would not confirm a claim by school principal John Edwards that police were also investigating two more former staff members.
Edwards said the two were named in a search warrants served on the school in July.
"One of them was a former teacher at the school and the other person worked in other capacities at the school," Edwards told ABC.
He said the warrant alleged the abuse occurred in the mid 1980s.
Goodwin declined to discuss details of the accusations.
However, Edwards told ABC he had received several years ago allegations of orgy-related abuse at the school and alerted police. It is not clear how police responded.