If you leave from BNE and return to SYD you are not allowed ANY stopovers as your itinerary has an open jaw - the rule is open jaw OR stopover.
I managed to book a double open jaw (i.e. A - B // C - D). I fly out of SYD (A) and back into MEL (D) only because there was no availability out of MEL. I asked politely if I was allowed to dp this (and the reason - I wanted to fly out of MEL but couldnt). The was allowed, but I think the operator thought MEL was SYD's second airport (she mentioned something about them being an airport pair)