Can i use my mobile phone to get help after a big flood?

There's an app: Yes, there’s an app for that... to get help after major disasters. You can download the Red Cross’ free Emergency App that provides real-time severe weather alerts and safety information. Text GETEMERGENCY to 90999 or search "Red Cross Emergency" in the Apple App Store or Google Play Store.

Floods: Floods are New Zealand’s number one hazard in terms of frequency, losses and declared civil defense emergencies. Floods can cause injury and loss of life, damage to property and infrastructure, loss of stock, and contamination of water and land. Getting ready before a flood strikes will help reduce damage to your home and business and help you survive. http://www.getthru.govt.nz/disasters/flood/