Re: *Would have been* valuable to residents
It's not an app.
It uses the government alerts feature built in to iOS (and, presumably, the Android equivalent).
http://www.civildefence.govt.nz/get-ready/civil-defence-emergency-management-alerts-and-warnings/emergency-mobile-alert/emergency-mobile-alert-frequently-asked-questions-faq/
https://support.apple.com/en-au/ht202743