The time of arrival of BA15 is not good and it is a shame that Qantas no longer fly via Singapore with the flight that used to arrive in Sydney at around 8 pm. If doing it all in one flight, this was the best option.
The other suggestions about going via HKG to get a better arrival time in Sydney make sense. I guess if the op has a flexible ticket then there is some scope for change but if not the advice about sleep sections, eating etc etc will help on this long flight. We have been in Australia a month now and it took best part of a week to get into the time and there is not much you can do to get around this.