Australia is crossed by plenty of long distance back roads or trails. Most are suitable for a well equipped mountain bike if you have a week or two to spare. All these trails need a reasonable level of fitness and some experience in bike touring self-sufficiency is useful.
Some routes were used by gold prospectors, Tanami Track, or follow the Old Ghan and Overland Telegraph Line, Oodnadatta Track. Others like the Gibb River Road or the Buntine Highway were development roads constructed to open up areas for cattle stations in the 1950s.
There are others that happen to traverse interesting country in linking destinations: Karijini National Park, Kakadu, Jutpurra (Gregory) National Park, Mereenie Loop, Finke Road and south coast of Western Australia.
The fabulous and challenging Cape York ride leads from Cairns to, well, The Tip at Cape York.
The Mawson Trail is a great introduction to the getting-off-the-main-road mountain bike expedition. It’s easy to get to, just start pedalling down outside the Festival Centre but then again getting back from Blinman is more of an issue. The Mawson is scenic, starts with plenty of services and budget accommodation nearby but then edges more remote. 2 weeks should be sufficient taster to encourage further little adventures.
The Munda Biddi Trail and Tasmanian Trail are roller coasters with plenty of huff and puff and the occasional push-a-thon and needing off-road mountain bike skills. The often rugged road surface through the Jutpurra (Gregory) National Park makes this a challenging ride in places.