AS1851-2012
Section 1.5.6.1
A critical defect is one that renders the system inoperative.
A critical defect has a significant and direct impact upon the safety of occupants of part, or all, of the building.
Examples of critical defects include an impaired water supply which is unable to provide water to a sprinkler system or an inoperative fire panel which is unable to warn the building occupants of fire and alert the fire brigade.