Baileys,
The system is not being held together just by overtime. There have been a number of policies put in place in the last couple of years, without which would have caused an airspace closure or a NOTAM issued identifying possible delays and restrictions.
Closures have been minimised through the "Short Break Procedure" where a controller may leave the operational position, to go to the toilet or eat something, as long as they leave written instructions for an unqualified person (usually a supervisor) to hold the fort until their return, and not to do anything outside of those instructions. The controller generally needs to pick a time where there is little chance of something happening. CASA has approved this. Previously, a NOTAM would be issued notifying TIBA for the period of the break, and all would be known by all parties well in advance.
Traffic management (delays, restrictions) still occur but instead of NOTAMing delays and restrictions due staff shortage or "staff availability", traffic is diverted or slowed down without any NOTAM or reason give. The practice of controllers who transmit the reasons for the delays (lack of staff) is frowned upon.
Other ways to avoid closures is to "reinterpret" the minimum time off between shifts, and acquittal hours due fatigue issues, thus maximising coverage where previously coverage wasn't possible.