John Sidney Denham pled guilty to more than 25 charges involving the sexual abuse of 18 boys at the school he worked between 1975 and 1979.
The 71 year old has been jailed for a maximum of around 20 years for the offences.
He told Sydney's district court: "I am horrified that I have done this.
"I was a self-indulgent, self-obsessive person who just wanted what he got."
One of the priests victims broke into tears as he described how the abuse had left him suffering night terrors, paranoia, insomnia and mood swings.
Another victim told how he was caned after he made allegations to the school's principal, also a priest.