Edge of Light Trilogy 3 Fire

We meet Merel, a girl living on a futuristic island in the middle of a sea. Many years have past since the community survived the floods and the severe heat that has forced their forbears to shadow their world. They live in twilight and their year is punctuated by traditions that address the need to restrict their population drastic traditions that are called the truth. We come to realise this is set in a future world that might be New Zealand.