In all states of Australia, motorists are required to keep left unless overtaking when a "Keep left unless overtaking" sign in displayed or on a multi-lane road with a speed limit of 90 km/h or greater.
Note that the rule in some states says "more than 80 km/h" which still means 90 km/h speed limits and above. Some states describe the rule a little clearer as "90 km/h or more".
There are certain exceptions to the rule. You can travel in the right lane if you are going to turn right, make a U-turn, are avoiding an obstruction, driving in heavy traffic or using a special purpose lane that you are allowed to be in (eg. taxi lane).
Does keeping left mean always driving in the left lane? The rule applies only to the right lane, so if there are three lanes the keep left unless overtaking rule does not apply to the middle lane.
Overtaking to the left of a vehicle on a multi-lane road is legal in Australia, but the rules state that it must be safe to do so.
It goes without saying that it is not legal to exceed the speed limit no matter how you overtake another vehicle.