QF have no interest in giving BA a piece of the pie for selling a SYD>MEL flight they have nothing else to do with because your other option is letting QF have the whole pie. For the most part codeshares such as these are only bookable if attached to a flight on which the codeshare partner is prime or at least a joint business route. i.e. BA can only sell (LHR-)SIN-SYD-MEL not SYD-MEL.