These kind of moves rarely happen as all those in the upper echelons have no interest in actually living in Western Sydney or commuting for 3 hours a day from the Eastern Suburbs or North Shore. They might move some operational sections to Western Sydney but the main departments in HQ will remain where they are or end up in the CBD.
Not forgetting the several large businesses in recent years who have nearly blown up because they decided to move HQ only to see all their staff resign as they didn't want to commute or move interstate.