By 'unwell' did you mean 'had been injured'...? Because according to Hallmark ground staff, apparently that was the case.
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.