The first thing I noticed upon ordering this book for my first trip to New Zealand last year is that this book isn't really very big. Especially for the money you pay for it in the UK. Secondly the majority of the book is taken up by pelagic seabirds and waders, many of which are rare vagrants. Thirdly the quality of the book is not terrible by any means but the illustrations are not the finest in the world. The text is adequate for ID purposes but presents little information.
Upon arriving in New Zealand I found that there really aren't that many species of non-marine birds there and many of those are introductions and will be familiar to UK birders. Of course when you do finally get to grips with NZ specialities like the Tui and the Kea and so on - they are all fantastic and unique birds. But heres the crux of it - you really don't need this guide in order to id them as they are all so distinctive and there is a lot of information to help you at any of the National Park or Dept of Conservation Offices. There are no complex warbler groups, only one pipit, only one lark etc. Really one of the small photographic guides (which I always think are aimed at the casual wildlife enthusiasts rather than serious birders) will probably suffice unless you want the most comprehensive and complete guide on the market. And if that is the case I would recommend the 'Field Guide' version of this publication which has all the contents of this book plus plenty more information to make it an informative read in addition to an ID guide - and its only a few pounds more. Therefore it kind of makes this book, good as it is, quite redundant.
Plus if you are really into pelagic bird spotting you can get a seperate and probably better book for them (though I haven't read it myself) plus you can get guided boat-trips with knowledgeable guides from places such as Auckland or Kaikoura.
New Zealand is the most beautifu place I have been to and I recommend it highly - not many species for hardened birders but the ones they have are gems and the scenery is spectacular to match. For birds I recommend taking a whale-watching trip from either Auckland or Kaikoura, the Otago Pensinsula at Dunedin (very good guided tours there), Tiritiri Matangi Island Sanctuary, Fjordland (go tramping along the Dart/Rees), and the mecca of NZ birding - Stewart Island, your best chance of spotting a wild kiwi (though sadly I didn't!).
But as I say you are better off investing in the 'Field Guide' version of this book if you are a serious birder/naturalist.