As a quick reference, when it comes to the depiction of minors, the Wikipedia article on the legality of child pornography comes in handy:
https://en.wikipedia.org/wiki/Legality_of_child_pornography
As can be seen under the Oceania section, possession of any child pornography (ie. pornographic material), whether real or fictional, is illegal in Australia. While ages usually go unmentioned, it's unlikely that, for example, first year high schoolers are going to be 18 or older.
As for other works that depict "obscene" acts such as rape, bondage, homosexuality, etc., you'll have to look up local laws, and I implore anyone to do the same, even if using the quick reference above for works depicting minors. This is especially important in federated nations, like the USA, Canada, Australia or Germany, where laws may vary from state to state or province to province.