Take the North Island through summer (you shouldn't have too much trouble with accomodation, unless you are in the peak tourist/holiday areas over the Christmas break ie - Bay of islands, coromandel, gisbourne, etc) and then the south island towards the end of summer and autumn. Parts of the south island would be too hot in summer (think low 30s, but in the direct sun it feels hotter than any 37 degree day i've had in Sydney) and the place is stunning in autumn. Come winter, its getting too cold.