Stick to MEL or SYD. If you like exotic stuff, head to the Blue Mountains outside of Sydney on an organized tour or by train (10,80 AUD).
in CNS, you can do the Great Barrier Reef for an entire day + go to Kuranda gardens where you can hold a Koala and walk through a real rainforest
Either way, stick to one place in Australia, as just getting over the jetlag (if applicable) + getting to and from the airport, ... Make the best of it