An investigation has been launched into allegations of child sexual abuse at one of Sydney's most prestigious private schools dating back more than 30 years.

St Ignatius' College, Riverview on Monday sent a letter to its old boys informing them that a former student had made allegations which principal Paul Hine said had now been reported to the NSW police force.

Former student lodges historic allegations: Sydney private school St Ignatius, Riverview.

The only detail of the allegation contained in the letter is that it concerned "child sexual abuse over 30 years ago".

Dr Hine said it was with "some sadness" that he wrote to the school community about the allegation which had been passed on to him from the professional standards office of the Australian Province of the Society of Jesus.