Assuming that pornography in general is acceptable, if Australia's governing body wants to be responsible about this then they should commission a study that covers whether small breasts in pornography affect pedophilia rates. If they do in a statistically significant manner, then they can use that as a legitimate reason to place restrictions on it. Otherwise, I think it would show two things: 1. that they have no interest in the facts of the matter; and 2. that they have no respect for the freedom of their citizens to do as they like.
"Words are deeds." - Wittgenstein