THE Royal Commission into Institutional Responses to Child Sexual Abuse has heard more than 20% of members of some Catholic religious orders were allegedly involved in child sexual abuse.

The commission in Sydney heard orders including Marist Brothers and Christian Brothers were involved, reports the ABC.

Gail Furness SC used her opening address reveal a survey which showed 4,444 alleged incidents of abuse between January 1980 and February 2015. 

She also said private royal commission session revealed 60% of those who attended reported sexual abuse at faith-based institutions, two-thirds of which were Catholic. 

Ms Furness called the victims' accounts of abuse "depressingly similar".

"Children were ignored or worse, punished," she said.

"Documents were not kept, or they were destroyed. Secrecy prevailed as did cover-ups."

In the first time the information has been released, it's been revealed the order with the highest proportion of allegations was the St John of God Brothers with 40.4 per cent accused.

More than 20 per cent of Marist Brothers, Salesians of the Don Bosco and Christian Brothers were also accused.

The average age of victims at the time of their alleged abuse was 10 for girls and 11 for boys.

The Archbishops of Sydney, Perth, Brisbane, Adelaide, Melbourne and Canberra-Goulburn are in Sydney to give evidence during the three-week public hearing. 

It is expected they'll be questioned on the extent of child abuse for the past seven decades and what current leaders are doing to protect children.

This public hearing is the 50th of the four-year-long royal commission.

It is the 16th dealing with abuse within the Catholic Church.