Tipping is not an Australian custom and in general most people do not tip. When they do it is often no more than the small change left over from the payment of the bill. However, the increasing number of foreign tourists visiting Australia and following their old customs, as they do back home, is to a point making the locals used to the concept of the gratuity.
On certain festive days, restaurants warn customers of a special service charge to be included in the bill, to reward the staff for their availability on those special dates. Again, there is no need for tipping. Having said that, if you are very pleased with the service, especially in an expensive restaurant, and want to tip, an amount somewhere between 5% and 10% should be adequate and appreciated.
In taxis it is not customary to tip, other than leaving the change to the next dollar.