I presume that you are from the USA.
I have driven all round your fine country (N to S, W to E, S to N to SW to N).
Anyway, we are no where near as populated as the USA however land mass size is similar (see attached link for map overlay). Our highways are allright in parts however they are no interstates thats for sure.
Provide an idea on what you want to see or experience and I am sure you will get plenty of replies.
Great ocean road would be a suggestion as a starting point.
Australian maps overlay provides idea of how vast this country is.