Bus travel is the most popular way to get around Australia. Coaches go virtually everywhere and many coach tickets allow you to hop on and off en route to your destination.
There is a fairly comprehensive network of bus routes with lots of competition on the more heavily travelled routes between Adelaide, Melbourne and Sydney and up the east coast to Cairns making it a cheap way to get around.
Once you start to head west from Adelaide, there is less competition between various bus companies and the distances between towns are much longer making it a less cost effective transport option.
Coach travel is split between scheduled coach operators such as Firefly, Premier Motor Services and Greyhound and backpacker buses such as Easyrider, Oz Experience and the Wayward Bus. Scheduled buses are usually cheaper and run more frequently but the specialist backpacker buses often go to out of the way places that you would otherwise miss out on. Unfortunately some backpacker buses are too organised and feel a lot more like tours, which goes against the whole spirit of backpacking and independent travel.