The Australian Disaster Preparedness Framework defines a severe and catastrophic disaster as an event that:

… is beyond our current arrangements, thinking, experience and imagination (… has overwhelmed our technical, non-technical and social systems and resources, and has degraded or disabled governance structures and strategic and operational decision-making functions).

So, in developing the advice to consider a declaration, the agencies have provided information on the following criteria—listen up—'historical analysis and recurrence; the concurrence and compounding effects and scale of the events'—as I outlined in my first answer—'demographics; weather impacts'— (Time expired)