BA is not very motivated to help you ticket this as LHR-MEL is on QF (BA stopped serving MEL recently). You might find BA to be much more helpful if you change your routing to LHR-SYD on BA. In any case, your routing is valid and within the rules. You can quote the 2nd entry to Asia for transit between Europe and Australia. The rule is prety clear but different from other RTW product rules, which adds to the confusion. The rules are available on the web (see the sticky) or in BA's computer system.