The Field Guide to the Birds of New Zealand is the best field guide to NZ birds available! It covers all birds normally found on the North and South Island as well as on Chatham and Auckland Islands and others. It describes the birds, their appearance "jizz" and moulting in words that everyone will understand and find interesting and provides decent maps of migration, habitat, breeding etc. Compared to other books on the market this IS the benchmark!
However, I find the colours a bit off - they appear to be a little bright compared to studies in the field - and the texts suffer from lack of proof reading from time to time.
A more in-depth description of jizz would also have been appreciated as many of the native and/or sea birds in NZ only show themselves briefly and, especially if you're just visiting the country on a holiday, many birds can be difficult to establish 100% in the field without prior exposure - or good field guide descriptions.
All in all, I do recommend this book over any other book on New Zealand birds available! It comes close to the best of the European field guides!