Priests in Canberra will be required to report allegations, offences or convictions related to children to the ACT Ombudsman within 30 days from March 31, 2019 - even if the information was gleaned in the confessional.

The laws passed the ACT's parliament last month, although the confessional clause was delayed for nine months so the government and the clergy can determine how the laws will apply.

Under church law priests can be ex-communicated for breaking the seal of confession.

Anti-child abuse campaigner Damian De Marco told Fairfax Media: "If they really gave a damn they’d be ex-communicated rather than let a child get raped".

But the Australian Catholics Bishops Conference wrote to ACT Chief Minister Andrew Barr with "grave concerns" about the laws, saying mandatory reporting from the confessional "will either have no effect on child safety or will actually make children less safe".

"Firstly perpetrators of this terrible sin very rarely seek out confession and if mandatory reporting of confessions were required they would certainly not confess. There would be no effect on child safety," reads the letter signed by Melbourne Archbishop Mark Coleridge and Sydney Archbishop Anthony Fisher.

"Secondly any perpetrator who was minded to confess would almost certainly do so anonymously so no mandatory reporting would be possible. Again children would be no safer.

"Thirdly were trust in the absolute confidentiality of confession undermined then any chance a victim would mention this in confession to a priest would also be seriously diminished; any chance a priest-confessor might have to impress upon the victim the need to inform responsible adults outside confession and get to safety would also be lost. Children would be less safe."

The archbishops said people of religious faith would be "scandalised" if their priests revealed what they had confessed to police or in court.

They wrote that the "spectacle of arresting and trying priests for acting in accordance with their faith would be very socially divisive".

"Cultures and polities like Australia’s rightly recognise clear distinctions between Church and State," the letter says.

"While the two necessarily intersect at various points, even collaborate, they rightly give each other a wide berth at other times.

"To require priests to break the seal of confession would be overreach by the state into the domain of the sacred.

"Freedoms of conscience, thought, speech, assembly, religious belief and practice are basic human rights.

"To force a priest to breach the seal would be to force him to act against fundamental tenets of his faith."

Mr Barr's letter of reply said the Royal Commission into Institutional Responses to Child Sexual Abuse received evidence that some perpetrator priests had confessed to their abuse in the confessional, and was satisfied the sacrament played some part in both the occurrence of abuse and its cover-up.

"For those men the confessional provided a site for them to acknowledge sin or wrongdoing and a space to ease their guilt after abusing.

"Child abuse is appalling and my government is unwavering in its approach to keeping children safe, wherever they are."

Mr Barr pointed to the Holy See's agreement to a request from the American Bishops' Conference for an exemption from the pontifical secret in order to disclose child sexual abuse.

He noted the royal commission had urged the Australian Bishops' Conference to seek a similar amendment.

However the conference has previously said it does not view the sacramental seal as "incompatible" with maintaining child safety.

"The Church wants measures that will genuinely make environments safer for children. There has
been no compelling evidence to suggest that legal abolition of the seal of confession will help in that
regard," president Archbishop Coleridge said in the statement.

The debates comes after Adelaide Archbishop Philip Wilson last week vowed to appeal his landmark conviction for concealing the sexual abuse of children in the Hunter region near Newcastle.