You can do that in 3 weeks. Head up into the high country, follow the Murray towards SA, drive up then to Tibooburra, then head off to Cameron Corner, Innaminka, back down the Strezlecki and do the Eyre Penisula after that, then come back via the Malley country, Grampians and Gt Ocean Rd. I've done most of that area and loved it. Gets a tad cold in the southern areas in winter though. Assuming you come from Victoria?