I was talking this over today,
What will happen if a RFDS pilot (or Ambulance chopper pilot or whatever) is tested just prior to a critical flight, they get a positive test (that later turns out to be false) and in the time it takes to get a new pilot the patient dies.
It's possible, it's the same situation as QF passengers being delayed but with more extreme consequences. If this happens CASA will have a lot of explaining to do.