I do this regularly and have found that the 19:00 flight is too stressful a transfer to QF9. I have had to run all the way from immigration to the furthest gate, whilst being harassed by staff to hurry up!

I usually get the 17:00 flight which leaves loads of time.

Best case you can get to the gate quickly but with a short delay on the BNE flight and a reasonable queue at immigration it can become very stressful very quickly!