The book I am looking for is a series. I grew up in Australia and while I cannot be sure, I think the author was Australian (possibly even female). The story is about three children or young teenagers. They live in the future where the world is too hot by day for anyone to be active, so they now sleep by day and live by night. The children have to go beyond their village or camp for some reason and they end up crossing vast land in search of something. I believe there were three books in the series, although it may have been more. I remember thinking it was interesting that everyone lived by night, with the light of the fire. The elders were respected. It was almost as though humans had to resort to the way of life before modern civilization. I think they have found a lost city on their adventure.

I have been trying for a really long time to find this book series. Any help would be greatly appreciated.