The basics seem to be that hitchhiking is illegal in Victoria and Queensland.
What the law states is that it's illegal to stand on the roadway. There is some discussion about whether you could stand off the road, eg. on the grass verge, and be ok. However I would not like to argue this one with a policeman. Technically it appears to be legal in other states, except on freeways.

There have been a number of bad things that have occurred to unwary tourist hitchhikers. For many reasons I don't think it would be a good way to travel. Australia is a large country, I would be checking out air and bus fares for travel.