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:
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you’re turning right or making a U-turn;
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you’re overtaking;
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the left lane is a special purpose lane, e.g. bus lane, bicycle lane, slow vehicle turnout lane;
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the left lane is a turning lane and you are going straight ahead;
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you’re avoiding an obstruction;
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you’re not driving a vehicle of a type that is prohibited from the right lane by way of signs (i.e. trucks);
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the other lanes are congested with traffic.
NB: On dual/multi lane roads where the speed limit is under 90km/h, the ‘keep left’ rule only applies where there is a ‘keep left unless overtaking’ sign.