Generally speaking, Australian banks are not Bitcoin friendly, and dealing with them directly is a major operational risk. Bank accounts can be frozen with no notice or even closed given only a short notice.
This is why Bitaroo chose to use a payment processor in order to mitigate these risks.
Our payment processor charges us a flat processing fee on each and every deposit or withdrawal.
Other exchanges recoup these costs by charging high trading fees. We chose to go another way.
All fees are detailed here.