An expert witness received loud applause as he savaged the Catholic Church for sacrificing the welfare of children at an emotional royal commission hearing today.

Dr Thomas Doyle, a United States canon lawyer and expert in areas of sexual abuse by clergy, has been giving evidence at the Royal Commission into Institutional Responses to Child Sexual Abuse in Sydney.

He claimed Pope John Paul II knew children were being sexually abused by priests and the Catholic Church attempted to cover up allegations.

Dr Doyle said victims and their families were coerced into keeping quiet about the abuse, even threatened with ex-communication.

"In the theology that I believe in there's no office in the Catholic Church or anywhere else is so important that it justifies sacrificing the welfare of one innocent child. Period," he said.

Pope knew about sexual abuse in 1980s

He said by February 1985 there were reports from at least four United States dioceses of priests sexually abusing children.

Dr Doyle was working in a Vatican embassy in the US and prepared a report on child sexual abuse in an attempt to bring the problem to the attention of the Holy See.

He wanted to gain permission from the Vatican to send a Bishop to Louisiana to address the problem.

"The report was sent by courier to the Cardinal Archbishop of Philadelphia, Cardinal John Krol, who took it the next day to the Vatican when he flew over," Dr Doyle said.

"He gave it to the Pope, and he wired back that the Pope did read it and that the appointment of the Bishop was made within three days."

The Pope at the time was John Paul II.

Commissioner Justice Peter McClellan listens to evidence in Sydney. ( Supplied: Royal Commission )

Dr Doyle said he chose Bishop AJ Quinn because he had a Doctorate in Canon Law, a Civil Law Degree and was "squeaky clean".

"He turned out to be part of the problem, not part of the solution, because he was trying to figure out how they were trying to continue with the cover-up," Dr Doyle said.

Staggering numbers revealed at royal commission

Yesterday, data collected by the royal commission and released for the first time painted a damning and systemic picture of child sexual abuse within Catholic institutions.

Gail Furness SC used her opening address yesterday to reveal 4,444 allegations of incidents of abuse between January 1980 and February 2015 were made to Catholic Church authorities.

Between 1950 and 2010, she said there were 1,880 alleged perpetrators from within the Catholic Church.

This is the 50th public hearing of the four-year-long royal commission and it is the 16th dealing with abuse in the Catholic Church.

The royal commission has investigated how institutions across the country, including schools, churches, sports clubs and government organisations, have responded to allegations and instances of abuse.