As I understand it -- and Monsieur de Marsup should correct me here -- 'backpacker' in Oz connotes a particular species of rootless, callow youth schlumpfing around, panhandling booze money, and generally avoiding responsibility. Sort of like Fort Lauderdale hangers-on, or the dopers at Goa. Or like the difference between "motorcycle rider" and "biker" in the US, perhaps?
Although how any country that celebrates its rovers-on-the-make and rates "Waltzing Matilda" the unofficial anthem can bust on slackers, we have no idea.

Must be a generational thing.