Catholic church authorities deliberately misused their own legal code to excuse claims of child sexual abuse and protect alleged perpetrators, according to testimony before a royal commission.

The fourth day of an inquiry into the Catholic church's approach to child sexual abuse heard that canon law has been used to justify the cover-up of alleged crimes.

In a submission to the Royal Commission into Institutional Responses to Child Sexual Abuse, Vatican adviser Baroness Sheila Hollins​ wrote that clergy may have used canon law to hide alleged sexual offending in their ranks.

"Canon law may have been deliberately misused to excuse inexcusable behaviour, and to cover up known wrongdoing," the member of the Pontifical Commission for the Protection of Minors wrote.