In Australia, different states and territories - and even different law enforcers - will approach surfboards mounted on your motorbike differently.
There is an ADR rule that the number plate must be clearly visible at an arc of 45 degrees from the surface of the plate to either side of the vehicle. Arthur is 179cm in length or around 5’ 11”.
Some websites state that your load must not extend more than one metre in front of the front wheel or more than one metre behind the rear wheel. In addition, your load must not be loose and must not touch the ground.