The abuse allegedly occurred at the prestigious St Stanislaus' College in Bathurst, west of Sydney, for about six years between the late 1970s and early 1980s.
"At this stage we're looking at 13 victims and we're following up those lines of inquiry at the moment," Detective Superintendent Michael Goodwin said.
Goodwin would not reveal how many suspects were being investigated but said there was no one teaching at the school at the moment who was under suspicion.
A 65-year-old former priest, who reportedly worked at the school in the 1980s, was arrested in May and faces 33 charges relating to sexual assault and gross acts of indecency on juveniles aged between 10 and 18.
School principal John Edwards said that he was served with a search warrant last month which listed the names of three former staff members.
He said he had first heard about the allegations some years ago when the college received some "Internet material" which included claims of late-night prayer and chanting sessions in which boys were sexually abused.
He said he had immediately referred the allegations to the police.
"Those circumstances where people have betrayed trust, it is a matter of grave concern and shame, and it needs to be addressed comprehensively in order to protect children in the future," Edwards said.
One of the alleged victims told the Daily Telegraph newspaper that his abuse began when a priest took him aside for private tutoring on religion. It later occurred in group chanting sessions during which students would pass out.
"They got a group between eight and 12 of us together and they'd just start chanting and I would wake up during these sessions and see what was going on," he said. "It was like an orgy."
The allegations come just weeks after the leader of the world's 1.1 billion Catholics, Pope Benedict XVI, publicly expressed his "shame" over the "evils" of clerical child abuse during a visit to Australia.
Benedict told a mass for clergy in July he was "deeply sorry" for the abuse of children by predatory priests and called for the guilty to be punished and for victims to be compensated.
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