The South Australian Catholic Church will not adhere to a change in law requiring priests to report confessions of child sex abuse, the Acting Archbishop of Adelaide says.

Under the new law, set to take effect in October, priests who hear confessions about child abuse will have a legal obligation to report the matter to police.

"Politicians can change the law, but we can't change the nature of the confessional, which is a sacred encounter between a penitent and someone seeking forgiveness and a priest representing Christ," Bishop Greg O'Kelly told ABC Radio Adelaide on Friday.

"It doesn't affect us.

"We have an understanding of the seal of confession that is in the area of the sacred."

Canon (Church) law lays down that "it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner and for any reason".

The penalty for violating the seal of confessional is excommunication, as every priest knows. The law also stipulates that anyone else who happens to hear or overhear someone confessing sacramentally is also obliged to observe the seal.

Not reporting abuse will carry a maximum $10,000 fine, and brings expectations of priests in line with those of social workers, teachers, medical professionals and others in positions of authority.

An Attorney-General's Department spokesperson said authorities intend to follow up instances where the law has been broken, and prosecution may result.

"Where there is clear evidence to indicate a minister of religion... has failed to abide by their mandatory reporting requirements, the matter would need to be investigated by authorities, with further action - including prosecution - taken as appropriate," a statement read.

The law forms part of the South Australian government's response to the Royal Commission into Institutional Responses to Child Sex Abuse, released by Attorney-General Vickie Chapman on Tuesday.

Bishop O'Kelly said the church had not been made aware of the change, which was legislated last year, until Thursday.