As a rule, tipping is not necessary in Australia, wages are sufficient to make a living anyway and this shows in the eye-watering prices for any service. Most cafes and casual restaurants have tip jars, where you're welcome to reward good service with a coin or two (or, like us when eating out with our two-year-old, apologize for mess), but this is purely voluntary. Likewise, you probably would not tip your cabbie unless he eg. carries your bags for you or finds a magical way through a traffic jam when your wife is having a baby in the backseat (not that I would have stuck around waiting for change anyway...).
The only situation where tipping is kinda-sorta expected is in fancy restaurants with degustation menus, white tablecloths and sommeliers hovering by your elbow, in which case 10% is standard. The justification I've heard is that people in these places are more skilled than the grumpy uncle at the corner truck stop, yet get paid the same (union) wages, so the tips are their reward for going the extra mile.