Mspec:
I don't get the because it only has x people you don't need security but if it has over that it doesn't
What does the ATR mean sorry don't know is it the type of plane.
I comes down to the aircraft size and also its capability, i.e. how much fuel might it be carrying, and how much range it has. In other words could it be hijacked and flown to Australia to commit a terrorist act there. Also it would be very expensive to have security at every provincial airport. Therefore it was decided the most practical choice was to screen jet services only.
However as other have posted this security thing is a pretty pointless exercise. It is a feel good, waste of the travelling public's money.