New Zealand is a wonderful place for kayaking and canoeing. Glorious sun-soaked beaches and inlets make for interesting sea kayaking, and inland it is more mountainous than Australia, making for some of the worlds best whitewater rivers.
There is an excellent guidebook to the New Zealand White water rivers, but it does tend to be aimed at those paddling fairly hard. New Zealand rivers have big water, so if you are used to bouncing your way down British rivers between the rocks, take a conservative attitude to what you expect "grade 3" to mean - it can mean waves bigger than you've ever seen before !
South Island's [Buller River]? catchment offers a huge range of trips from grade 2 upwards. The New Zealand Kayak School in Murchison is the place to hire gear (or take a course) - extremely friendly and helpful. Remoter trips on bigger and longer rivers require access by helicopter, but around Murchison itself, there is "boutique" paddling where you can do a river, stop off at the café, and do another river ...
New Zealand whitewater boats are very similar to those elsewhere (indeed, many are from the same makers), so the European or American paddler can quickly feel at home.
The whitewater guide doesn't cover touring rivers. The New Zealand Recreational Canoeing Association publishes a detailed guidebook to the [Whanganui River]? (a five-day 90 mile mostly grade 1 tour), but it is really hard to get hold of from outside NZ. There are other touring rivers, but information is probably even harder to find.
There are two nice guidebooks for sea kayakers, one covering the upper North Island and one covering the South Island. The latter doesn't, however, cover Fiordland?, a magnificent area with opportunities for some remote kayaking in the shelter of the fiords (which can, however canalise the winds, and often have nowhere to land for long distances). Beyond the fiords lies the [Tasman Sea]? and some very exposed coast with little or no habitation and very poor VHF radio coverage.
New Zealand sea kayaks are less familiar to those with a European paddling background, especially the doubles. Beamy boats seem the norm, with doubles up to 85 cm beam. Rudders are de rigeur, and skegs virtually unheard of. A lot of hire equipment is plastic, and the paddles are typically short and heavy (especially when you are swinging them over a boat of aircraft-carrier beam) and left-handers seem very poorly catered for. The market seems very set up to make absolute beginners feel stable, and for anyone new to the sport to be able to adapt to a right-feathered paddle. If you are a skilled, left-handed paddler wanting to do long tours in a nice narrow boat with a lightweight paddle, then don't expect to use hire gear that you can find on the net...
New Zealand's peculiar legal contraints can also make hiring equipment for children a bit of a nightmare - at least if you are trying to organise things via the internet.
There are skilled paddlers in New Zealand, of course, and a community of wooden boat builders too. If you are wanting a more familiar paddling experience, it would be well worth contacting one of the groups such as the Wellington or Canterbury Networks (need to add links).