There is a formula used by AsA to work out staffing requirements. This formula takes into account training, rec. leave, etc. It does not take into account sick leave or long service leave. This formula indicates that BN Centre is actually overstaffed??
When training comes up that has not been planned for, eg. Option 3 in NAS, this then means a lot of overtime to both fill controller seats and also train controllers. There is no surplus of controllers.
Also, if we go through a period of high sick leave, say in July/August, the Flu season, any surplus is quickly accounted for, and ED and Overtime is used to man consoles.
There is little, if no "fat" in ATC manning to account for such contingencies without relying on overtime or EDs.
The ATC college is also understaffed. I recently visited, and the guys there tried to shut the door and keep me there. That's how desperate they are for instructors!
BN Centre runs on the bones of it's arse. Every roster has been trimmed of any excess staff, and overtime and ED is used to fill shortfalls. Consequently, if a lot of controllers go sick at about the same time ( and this does happen from time to time), then you'll see airspace go TIBA.