If you didn't notice, Australia is a huge country, which means two things: firstly, there is going to be more that can be explored in a reasonably long trip, and secondly, it might mean you have to do a little traveling to get to points of interest.
South of Melbourne is the large island of Tasmania, a distance of 240 km (150 miles) over sea. Although there are regular flights, there are plenty of ferry and other boating services available that take anywhere from 9 to 12 hours. Either way, you should find some way to get there.
Once in Tasmania, head for a lesser known haven for adventurers called Eaglehawk Neck. Located on an isthmus on the eastern side of the island about an hour's drive east of Hobart, this tiny village is approximate to a number of interesting natural features. Head down to Pirates Bay, where it's popular to surf the breaks, and you'll find tessellated pavement, where erosion has given the rocks a sort of translucent linoleum look that is uniquely beautiful. There is also a blowhole near the beach where the ocean shoots through. In town, you'll find a community market, a dive center, and a center for fishing excursions. if that wasn't enough, the views looking down from Martin Cash's lookout are stellar. If you've got two days to get out from Hobart, give Eaglehawk Neck a visit, you won’t soon regret it.