Its possible that the same VS aircraft could not be used for both LHR-HKG-SYD and SYD-SFO if there is a requirement that the Australian operator of the flight (Virgin Blue operating as Pacific Blue or Planet Blue etc) may have to use a VH-registered aircraft. QF re-registered VH-NLH when leasing it from BA.
If that's the case, then it may just mean that VS has to position a few aircraft in SYD for the service. I am not sure what fleet capacity VS has available for such operations. 2 aircraft would be required to operate 4 times weekly.
Of course they could also wet or damp lease a 744 from SQ. Now SQ may offer a good deal just to have their livery flying the route even if not with their flight number. That might throw the cat among the pigeons. But not sure the DJ management would go for that one
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