I believe VS can always get staff on the HKG-SYD segment, but the LHR-HKG segment is always nigh impossible. There is a huge demand for LHR-HKG-LHR so that demand often blocks any through traffic wanting LHR-SYD.
I would suggest getting to HKG on maybe LH or SK or OS etc etc and then down from there.
The alternate is a ZED on CX and QF - between them they run 5-6 daily HKG-SYD.