The experts will investigate for many months, anything else is school yard speculation and has no place here.
One thing we know for sure, the trip was not
planned in accordance with the requirements of CAO 82.0. If the flight had been planned iaw 82.0, this event would not have happened.
The provisions of CAO 82.0 relating to "Remote Islands" were created specifically to prevent this sort of event from happening. The forecast weather conditions at the island do not come into the planning requirements of the CAO. It applies "whatever the weather conditions"
Simply, there should be enough fuel to fly from the departure point to Norfolk and onto an alternate that is not itself located on a remote island.