DHC4 burns around 600-700kg between SY and CB.
Reason Qantas have only a few jet services a day to CB is even with full loads B737 makes no money between SY and CB, where a full Dash 8 makes heaps. The jets are only there to keep those that just have to fly business class happy, and position aircraft to do other services out of CB.
Even with lower operating costs, such as Virgin or NJS, I would guess very little profit would be made, even with every flight full.