On QF domestic flights (QF400 and above) seat assignments are done the night before travel by the QF computer system based on preferences in frequent flyer profiles. The systems works a treat and ensures that eleite passengers booking up until the day before travel are able to be alloctaed their seating preference.
Note that a flight BNE-SYD is about 1 hour flying time so there really isn't much in the way of bad seats.
Make sure you have a valid seating preference in you FF profile (for any OneWorld carrier) and that should suffice. Otherwise turn up early for check-in.