Perhaps the bigger question should be 'why weren't there any other F/O's available to operate the flight?' - which lead to the schedulers trying to shaft 2 F/O's that arrived on the morning of the delayed departure to operate back out again that night - a bare minimum 12 hours after touching down on arrival from SYD/MEL.
Geeeze. Since when did any airline have stand-by crew, or a recovery plan, at out-ports to cover the possibility of occasional illness and other adversities?
Get real guys - is this another example why pilots should stick to the flying?