The special commission of inquiry into allegations of child sexual abuse by the Catholic Church in the New South Wales Hunter Valley has been formally opened.

The inquiry is separate to the national , and was announced by the New South Wales Government last year.

The State Government made the move after Detective Chief Inspector Peter Fox went on the ABC's Lateline program to say the Catholic Church and police were guilty of a cover up of sexual abuse by priests in the Maitland diocese.

Opening the inquiry, Commissioner Margaret Cunneen SC spoke of the devastating effect sexual abuse has on a child by exploiting their vulnerability.

She says it is very hard for children to talk to adults about sexual abuse.

"When they do, the collective responsibility to take action weighs heavily on all," she said.

But she says in this case it appears that responsibility was ignored.

Commissioner Cunneen says two priests, who have since died, victimised children.

One of them, Father James Fletcher, died in jail, but the other Father Denis McAlinden was never dealt with, even though many in the church accept he was guilty of four decades of abuse.

One bishop has said he, "should have been dealt with earlier."

The public hearings begin in May, and any victims who have not yet contacted the Commission are urged to do so.