I have lots of recs. *grins*
To start with, Winter of Fire, by Sheryl Jordan. It's a post-nuclear-winter (implied, never stated) where a group of people calling themselves the "Chosen" live off the coal mined by the "Quelled". It follows the adventures of a young Quelled girl. It's very beautifully written, in my opinion, lots of good imagery.
The O trilogy, by Maurice Gee - The Halfmen of O, The Priests of Ferris, and Motherstone. In Halfmen, Susan is kidnapped into another world, entered through an old mineshaft, and her cousin Nick follows her. They find a land where every human has been forcibly made only good or evil - and, naturally, only two of the good survive. Susan has to reunite the two Halves and make the people of O whole again. Action/adventure fantasy, with well-drawn characters and a beautiful fantasy world. They have to be read in order, as they play upon each other, but the series as a whole is an excellent examination of consequences and beginnings. Mythological.
Under the Mountain, also by Gee. Two children, staying with their aunt and uncle in Auckland, have to stop a family of aliens from awakening the dangers under the volcanoes of Auckland and taking over the Earth. Unlike the O trilogy, which is Kiwi-inspired but not driven, this is set in modern-day Auckland and very much revolves around the geography and feel of the city.
The Changeover, by Margaret Mahy. Laura Chant's little brother is having the life drained out of him by a demon, and she goes to the enigmatic Sorenson Carlisle, a prefect at her school, for help - convinced he's a witch. This is, above everything, a romance - with no skimping around the issues involved, given the ages of the protagonists (14 and 18.) It manages to combine fantastic magic with the sheer mundanities of life in a lower-class suburb on the outskirts of an expanding city.
I could go on - New Zealand doesn't do adult sf/f, for some reason, but the sheer amount and class of YA sf/f here is heartwarming. The authors above are all prolific, and then there's also Ken Catran (the Deepwater and Solar System trilogies are perhaps _the_ NZ YA sf series) and David Hill (lots of things.)