My guess is they'd have one, but over time they paid less attention to it, and some of the equipment they'd have had sitting in standby to mobilise for that event will have fallen into disrepair, as staff saw it as something would never actually happen.
In my role at work we have plans like that for failures of major assets, and thankfully they get taken seriously.
Dams, major water and sewer arterials, treatment plants, documents covering planned response to scenarios like statewide blackout, bushfire, cyber attack, terrorism, catastrophic failure, etc. However they can still be pretty grim scenarios, as the perfect contingency plan is something that nobody would be willing to pay for given it may never be used. I.e. dual supply pipelines, fun redundancy in the sewer system, etc.