mate who is an International 747 captain told me his airline uses woomera as a waypoint / alternate. It allows them to reset the fuel reserves enroute to Melb
That's not nominating it as a alternate, that's just a flightplanning trick to squeeze more fuel out of your flightplan. If they did not have enough gas at Woomera they would be diverting to Adelaide.
Nominating an alternate means that you will be going there if you can't land at your destination. It's not an emergency just a diversion.