I cannot fathom how anyone can believe that there are large or small or even minuscule numbers of penitents who routinely confess civil crimes in the confessional. I would guess that most priests would say that it simply does not happen.
In any case, anything that a priest hears in the confessional is simply hearsay. He takes the penitent at face value. The penitent may be confessing real sins, made up sins, or imagined sins. The priest has no way to evaluate whether the penitent is being truthful.
A penitent might even identify himself as someone else just to get another person reported to the authorities.
A law requiring a priest to report hearsay confessions to authorities would result in NO crimes being prosecuted. All that would happen would be that police departments would send fake penitents to make fake confessions and then arrest priests for failing to report the fake confessions. Maybe this is what Australian authorities really have in mind.
And none of this addresses the fact that according to Church law, every penitent has the option to confess anonymously behind a screen. They don’t have to identify themselves.
So what’s the point? The Church is not an agent of the police, nor should it be.
(I recall reading about an incident wherein one of the kings of France suspected his wife of adultery. He demanded her confessor tell him what she had confessed. The priest, of course, refused to say anything at all about the confession, and was put to death. The wife, incidentally, had always been faithful. Sounds like this may be where Australia is headed.)