In reply to Chris Fryer:
It is actually a bit of a bummer to try and combine both snow and climbing in one trip (of only 3 weeks). The main reason for this will be the weather! If you come in Spring, it is likely to be windy and wet, although the nice days are v v nice! You can equally get great days on the mountain and on the rock, but it will be a bit hit and miss for both and you will probably (unless you are very lucky) have days when you can't do either.
Winter tends to be drier and slightly less windy and could be your best bet. The snow on the mountain will be good (although this year it has got better and better as we got closer to spring), and the weather off will probably be dry. However it will be cold and you could find there is snow everywhere rather than just on the mountain. This however doesn't tend to last that long at sea level so you'd probably only lose a day or so.
Summer is by far the best time to see NZ, but you are less likely to get any boarding in, although I think it is possible that Ruapehu will be open for summer skiing this year.
The problem with NZ when trying to plan a trip that is subject to weather is that you just can't predict it. It is an island with fast moving weather systems passing over it that tends to get very affected by both the weather in the Pacific aswell as what comes off the large land mass of Oz. My parents came during what is supposed to be the best month of the year weatherwise and we had a cyclone! My sister came when it was supposed to be comfortably warm but not as sweltering as summer and had ice forming on the windows in her hostel and had to go out and buy a new sleeping bag. The week later it was back up to double figure temperatures. It is just one of those things which can really ruin any plans.
You therefore need to come with and open mind and be willing to be flexible about what you do. I'll add a bit about the snowboarding specifically in a mo, but I think it is safe to say that the boarding and climbing in NZ is not that amazing and I would never recommend someone comes to NZ just to do one of those or even both. However, if you come to see NZ and happen to take in those activities you will enjoy it. The most amazing thing about NZ is the variety - the fact that you can get from craggy coastline to desert sand dunes to rain forest, to teletubby hills to snow capped mountain to huge surf within a few hours drive. Everything is on your doorstep here and it is amazingly unspoilt compared to most of the world.
If you want any specific info let us know. We are more knowledgeable about the North Island, but have travelled the south island quite a bit so can help there too.
Climbing venues which I'd recommend are Castle Hill, Paynes Ford (north of South Island - never been but heard it is pretty good) on the South Island and Whanganui Bay (stunning venue) and Wharepapa (good scene centred around Bryce's) on the North Island.
This season (on snow) has been an amazing season with the North Island mountain having over 3m until this weekend (when it rained and washed over 30cm of snow away). We have season passes and have been down there throughout the season with our first weekend being at the end of June. At that point, the upper mountain was loads for fun for boarding - there was 2m of snow but due to the rocky terrain there was an abundance of features to play with. The down side was that lower down the mountain, the rocks were pointing through a bit too regularly so a lift down to the bottom was preferable. By mid July however, there was close to 3m of snow and the rocks had disappeared. So had many of the great features I was enjoying earlier in the season however, although it was still fun. The weather in July was relatively reliable and I don't think we missed a day on the mountain due to it being closed (only go down at weekends).
August saw lots of snow but as a result the mountain was closed much of the time due to weather.
September saw fantastic snow and reasonable weather but later in the month and running into October the weather started getting more unpredictable. Spring on the north island tends to be rather wet and windy which has actually brought lots more snow to the mountain, but also alot of closures!
The mountain is however open until 21st November this year (for the winter season) with a fairly large possibility of it opening again for summer skiing around Christmas time.
The south island slopes, while having great snow, have closed earlier than the north island due to lack of custom. There just isn't enough people down there to justify staying open.
As someone who has boarded in Europe and Canada, I have to say that NZ isn't amazing for boarding. It doesn't have the huge powder fields that Canada has to offer and not really as many features (when there is lots of snow), however this season has seen some amazing powder days (rather annoyingly all midweek) and the runs are shorter and more fragmented than the European resorts. However the atmosphere is really cool, the mountains deserted (relatively speaking), and it is so close to everything else - 4 hours drive from Auckland on a Friday night, oh and v cheap for a season pass
You can surf, ride and climb in the same weekend withouth much effort!
If you want any specific info, just email me or Matt.