The problem is each state has variations in the rules. Really time for national road rules, but not just adopting NSW's rules either.
In WA the keep left rule only applies in multi lane carriageways with a posted speed limit above 90kmph. 90 or below you can travel in either lane.
Above 90 eg highways and freeways becomes a little more clouded. The road rules state you must keep left when overtaking full stop and state you must keep as far left as practicable. In my interpretation that would be the far left lane. But there are legal exceptions to this as outlined in the WA road rules:
Penalties
Failing to keep left in a multi-lane road =
2 demerit points and $50 fine.
On single lane roads you must drive as far to the left as practical (except motorcyclists). On multi-lane roads if the speed limit is 90km/h or more you must drive in the left lane. This same rule applies to any road where there is a ‘keep left unless overtaking’ sign. On these roads you can only drive in the right hand lane if:
- you’re turning right or making a U-turn;
- you’re overtaking;
- the left lane is a special purpose lane, e.g. bus lane, bicycle lane, slow vehicle turnout lane;
- the left lane is a turning lane and you are going straight ahead;
- you’re avoiding an obstruction;
- you’re not driving a vehicle of a type that is prohibited from the right lane by way of signs (i.e. trucks);
- the other lanes are congested with traffic.
What also may be open to interpretation is when are you actually passing eg. Is it safer for me to stay in the right lane when sitting on the speed limit and overtaking slower traffic with gaps of say 100m between cars? Or should I continually change lanes and end up weaving? IMHO the first option is the safer.