Yeah Ive done it. Not really that much backpacking though. I went there with two friends for 3 months. We stayed in Sydney and worked meanwhile, well at least two of us did. I worked for about a month in Sydney and then we bought a van and drove around cross country. It was so cool. A very good website to find condos, jobs, cars and what else is http://www.gumtree.com.au/
Its kinda like craigslist.
Places you should visit are definitely Byron Bay, Brisbane, The Gold Coast, You should do some surfing aswell. Nimbin is pretty fucked up. If you want to go there it will be good to have friends with you. If you want to smoke weed buy it and then fucking leave.
We just travelled around the East Coast of Australia but depending how long you want to stay you could visit Perth, Melbourne and maybe hit up the hot North. Many people visit Canberra aswell but its not impressive or anything.
Depending when you want to go you have to consider its the opposite season. So if you go in Summer it will be winter in Australia. It was like this when we went and it was awesome. I went skiing at Perisher in the South and then we headed up the coast and went surfing. Haha.
So maybe think about bringing your boots and some old skis or something if you want to.
Inbox me if you want any more info, travellers tips or you just have a question.