Clunk Straight from Queensland Department of Transport and Main Roads, no mention about being allowed on the roof either.
I am guessing here, but having lights at roof height / roof rack mounted would probably create some reflection off the bonnet. I read this in a previous post - The lamp/s must be installed in a way that the light produced does not cause the driver of the vehicle discomfort either directly or by reflection.
Years ago, more than twenty come to think of it, I heard / read that spot lights weren`t allowed to be mounted on top of bull / roo bar. Some thing to do with being in an accident and or some thing else to come in to contact with the spot lights being dislodged from their mounting point, causing the wind screen to be smashed. It could cause the driver to have an accident and not being able to have a clear view of where the driver be going.
My two cents worth, if it`s worth that much.