Are you trying to book using QR Qatar or BAEC avios? Or just what works.
Consider purchase the QF AU domestic flight separately and allow plenty of time to BNE/MEL/ADL/PER
D to I far easier at MEL & ADL than BNE & PER.

To me PER is more of a risk due to distance/time of the AU domestic flight and fewer flights per day.
SYD-MEL is a commuter run: flights every 20-30 minutes late afternoon.

Edit
11 March is a Saturday so less SYD-MEL flights.

Qatar QR's main AU domestic frequent flyer partner is VA Virgin Australia.