Don't be afraid of the SYD-MEL leg if you book oneworld and do the last leg as SYD-MEL: Qantas runs so many flights between the east coast cities and their irregular operations are very efficient. If you're delayed you just go to a transfer desk and Qantas staff will happily just issue you BPs for the next flight you can make and move you.
This was my experience booking through BA, flying with a random smattering of BA/CX/QF codeshares.