LHR-HKG-SYD is 2 miles longer than LHR-SYD (while LHR-SIN-SYD and LHR-BKK-SYD are over 100 miles longer). One of the rare cases of an intermediate city happening to be on the great circle route (of course the great circle is not actually flown on this route, for many reasons). The 400 extra miles must be eliminated by changing the routing, it cannot be paid for. There is a hard cap of 35K miles for Oneworld awards on QF and zero leeway (in theory). The likely solution is to take 400 miles out of the LHR-RAK-LHR flight, maybe going to casablanca instead (but I doubt that is close enough).