Allegations of child sexual abuse at a prestigious Sydney boys school, which were reported to police this week, have been known about by the body that governs the school for more than a decade.

The allegations have been made by a former student of Saint Ignatius College, Riverview on Sydney's lower north shore and date back more than 30 years.

The ABC understands the allegations have come to light now because the alleged offender is facing charges over similar matters in South Australia.

The principal of the private school, Paul Hine, wrote to the old boys yesterday informing them a former student had made the allegations.

But the Australian Province of the Society of Jesus, which governs the school, learned of the complaint from the former student over 10 years ago.

In a statement, the head of the Australian Jesuits Father Brian McCoy said the body encouraged the former student at the time to go to the police, but he chose not to.

He said the complainant was referred to Towards Healing, an agency of the Catholic church which was developed to support those who have been victims of child abuse.

"When the complaint came up in Sydney, we - my predecessors - referred the complainant to Towards Healing, which the Jesuits joined about the same time," Father McCoy said.

"We encouraged him to go to the police but the complainant, as I understand it, chose to not press charges."

The school was told of the allegations four weeks ago by the Professional Standards Office of the Australian Jesuits and the principal, Mr Hine, took the step of informing the students and alumni yesterday.

"Rather than sit on that, we thought it necessary to go out to our community in the event that anybody else had been adversely affected in the past by anybody involved in this school," Mr Hine said.

Mr Hine said the school's policies mean no current students are at risk.

"The current environment, it's absolutely inconceivable that anything like this could happen," Mr Hine said.

Mr Hine said the matter had been reported to police and the school was assisting the Jesuits' professional standards office with its investigations.

In the letter sent out to students and alumni on Monday, Mr Hine wrote he would also tell the boys at the school about the allegations.

"I am also communicating with all the boys today, in an age-appropriate manner, so that they do not hear about this matter in a piecemeal way or in a situation where they have no reassurances," he wrote.

He said the school took its duty of care seriously and he believed procedures at the school to safeguard students were of the highest standard.