AUSTRALIAN ROAD RULES - REG 140
140—No overtaking unless safe to do so
A driver must not overtake a vehicle unless—
(a) the driver has a clear view of any approaching traffic; and
(b) the driver can safely overtake the vehicle.
When can I overtake another vehicle?
Where the centre line marking on the road is a single broken line, vehicles may cross the line to overtake when it is safe to do so. If the centre marking has two lines you must not overtake if the line closest to your vehicle is unbroken. Where arrows are painted on the road, you must only drive in the direction they indicate.
Can I overtake road trains?
Road trains or articulated trucks (which can be as long as the length of 10 cars) can take up to 2.5 kilometres (1.5 miles) to overtake if you’re travelling at 100 kilometres (62 miles) per hour. Allow plenty of room before you try to overtake a road train as they often sway from side to side as you overtake. Be prepared for the wind-rush when passing as it can pull you towards the road train.
Overtaking other vehicles can be dangerous. The difficulty with overtaking is judging the space required to complete the manoeuvre safely.
Both single lane and multi-lane overtaking manoeuvres involve risk and require extreme care. If you have any doubts it is best not to overtake and to wait until it is safer.