Basings cheaper
It's probably cheaper for them to px someone both ways from SYD and do the regional flying out of HKG given the pay and package differential. This is something that I think was missed on the recent basings agreement. There should be a clause in there that limits any base to the amount of credit hours being flown directly from that base with a weighting for leave & sickness. This would avoid the sort of rostering that we're talking about here.
As pointed out by backspace this is happening on the Classic already, simply to avoid basing any Classic pilots in Hong Kong.