Australia is affordable by Western European and American standards, but certainly not a budget destination compared to say Southeast Asia. Your biggest costs will be accommodation and transport.
If you’re a midrange traveller hiring a car, seeing the sights, staying in hotels and motels, and enjoying the fabulous food and grog, budget for $110 to $160 per person per day. In cities you can push that figure up by $50 or so, but in less-touristed areas you can reduce it by around $30. Escalated petrol prices make multi-week road trips in a 4WD an expensive affair, but small, economical 2WDs are still wallet-friendly.
At the low-cost end of travel, if you camp or stay in hostels, cook your own meals, restrain your urge for entertainment and move around by public transport, you could probably eke out an existence on $70 to $80 per day; for a budget that realistically enables you to have a good time, aim for $100 per day.
