We have a prevailing west wind in NZ. The West Coast, especially in Te Waipounamu (the Sth. Is.) is therefore mild and wet. It can really pour with rain and then steam in hot sun from October to May. Water in rivers is pretty reliable year round. Jan/Feb is a great time to go there.

 To me, the jewel of 'the Coast' is Punakaiki (http://www.punakaiki.co.nz). Lush bush and limestone tumbling into crashing sea, expansive beaches and eery bushfed rivers. This kayak trip rocks http://www.riverkayaking.co.nz.

It's pretty busy there in summer, is all. 

You  can raft further east in Canterbury http://www.rafts.co.nz. It's in the rainshadow of the alps so can be dry in summer. And there's http://www.rafting.co.nz/ in Queenstown. Queenstown is spectacular, cool, crowded and expensive.

Where the water will be depends on climatic vagaries which seem to be getting less predictable. Ask me closer to the time and I'll check with my river-rat buddies.

 
Hey travellers,

 
News is that all the south island rivers are flowing well. If you are still keen on rafting there are many options. Canterbury, Queenstown and the Buller (Murchison) are good for one day trips with decent water and scenery. The best stuff is multi-day trips on the West Coast with heli-access. Stunning scenery, way in the middle of nowhere and great water (Karamea is recommended).

 
Dan