For an inbound flight for SYD we carry either CB, BN or ML as ALTN, depending on actual forecast when the FPL is prepared (15-18 hrs before ETA), and Captains decision. Our FPL's always tell us the fuel required for at least 3-4 ALTN's, to facilitate fuel decisions by the crew. Also due to coming in from the NW monitoring would enable numerous Tech Stops on the way , even once in the Australian FIR (think DN, AS, AD or BN/CS depending on inbound routing) We have a 'commitment to destination' enabler in our OM-A, however you couldn't use it for a Cat 1 airport with FG!