Sydney police have arrested a former teacher and a Catholic brother over alleged assaults against children dating back more than two decades.

Detectives from the Sex Crimes Squad arrested the 58-year-old former teacher in Blacktown in the city's west late on Monday afternoon.

A 59-year-old Catholic brother was arrested an hour later at The Entrance on the NSW central coast.

Both are being questioned.

Police allege the offences involved two 13-year-old boys in 1987 and an eight-year-old girl in 1985, with some of the incidents taking place at a Catholic college in Blacktown and a primary school at Lalor Park.

ABC TV's Lateline program reported the arrests are linked to an alleged paedophile ring, and it understands more arrests are to follow.

One of the victims, now a senior serving police officer, told Father Kevin Lee about the alleged abuse at the Patrician Brothers' College.

Father Lee claimed a paedophile network operated at the school with at least four members.

"You can't possibly have four in a school doing something against children and none of other staff being aware of it."

The accusations come as on the day the Government announced a royal commission into child abuse.

While welcoming the move, Father Lee said it was too broad and should be more focused.

"First the Catholic Church and the hierarchy and the different places that we know that police have been investigating," he said.

"And then we'll broaden it to look at other areas as they might unfold."