One controller was unfit for the night shift in Perth. The other absences did not impact negatively on the night shift replacement/availability.
One controller extended his 0630 shift to come in at 0200 OR the closure/TRA would have been 6.5 hours long. Unfortunately this point has been lost in the media beat up. For a very long time Perth controllers have covered the staffing shortfalls so that occurrences such as this have not been more frequent. However you can only do a certain amount of O/T and remain physiologically fit to do the job.
The night shift has only one person rostered between 2300 and 0530 local. The absence was medically cerificated and the controller would have breached the CASRs had they attended for duty. Single person night shifts rely on everyone being 100% fit 100% of the time, or you have airspace closures/changes.