One way: you cannot be tried twice for the same crime, unless under appeal (by either party). How could someone be tried in our courts, then in an additional sharia court, without watering down of the existing laws. Or if someone was convicted in a Sharia court and then themselves appealed their sentence, how could an existing Australian court uphold a ruling that is outside their scope of penalty?
It might sound good, but running on two differing legal frameworks would be fraught with problems.