Use the word gratuity and you'll get blank stares. There's no need for a "tip", but if you pay by credit card you will get hit with an extra 5% charge on top of the metered fare.

There's also an ~A$6 toll. You can choose to avoid that - normally it'll work out about the same price, but on Christmas day it'll probably be a little cheaper although slower to avoid the toll road. (The alternative is shorter distance and no tolls, but has traffic lights and slower speed limits)