The Catholic Church's response to allegations relating to a paedophile former priest from the state's north will be investigated by the Royal Commission into Institutional Responses to Child Sexual Abuse.

John Joseph Farrell, 62, was sentenced to a minimum of 18 years in jail earlier this year, after being found guilty of 79 child abuse offences against 12 victims.

The assaults were committed against boys and girls around Moree, Tamworth and Armidale in the 1970s and 80s.

The Royal Commission will now hold a public hearing in Sydney looking into the response of the Catholic Diocese of Armidale and the Catholic Diocese of Parramatta to the allegations of child sexual abuse made against Farrell.

It will also look at the response of the Special Issues Group for the Province of Sydney.

The brother of one of Farrell's victims has welcomed the investigation.

"The actions of certain members of the Catholic Church at the time allowed him [Farrell] to go on and abuse and destroy the lives of other children, and that really needs to be looked at," the brother said.

"All the victims and their families have always looked beyond his actual crimes, and it's going to help them, it's a necessary step.

"That legal justice step needs to be explored for any of the families, or the victims."

Bishop says victims are 'uppermost in his thoughts'

The Diocese of Armidale said it would cooperate fully with the hearing, and had already provided files and records.

Armidale Bishop Michael Kennedy said in a statement the victims of Farrell's abuse were uppermost in his thoughts and prayers.

"These victims have suffered greatly over many years," the statement read.

"While it is very likely this public hearing will be a difficult time for his victims, I hope that it will also be a time of continued healing for them as a light is again shone on the dreadful wrongs committed by John Farrell and the way the church let victims down."

The public hearing will begin in September.

Bishop Kennedy said the impact of the further investigations would impact on many.

"I am mindful that, even if not directly affected, parishioners, students, parents, clergy, religious staff and volunteers may be disturbed by what is reported during the public hearings," he said.