The traffic rules require that the driver being overtaken facilitates a safe overtaking:

'When a driver intends to overtake on the left, the driver in front shall stay as far to the right as possible and not increase his speed. If the vehicle being overtaken is moving slowly or occupies a great deal of space and the road does not offer an unobstructed view or there is oncoming traffic, its driver shall if necessary pull in to the right and slow down or stop.'