Remember any road that is over 80 km/H you must keep left unless overtaking or roads that are marked keep left unless overtaking, it is against the law to sit in the right lane. Think about other people and emergency vehicles. On multi lane roads left lanes are designed for slow vehicles or merging traffic, Centre lanes are flowing lanes and the far right of the the lanes is an overtaking lane, once the manoeuvre has been completed, merge back to the centre lane or the left lane.