Murchison is definitely a good place to base yourself out of for paddling. I spent 2 weeks at the campground (holiday park), located on the river just north of town, and hooked up with other visiting boaters. A rental car, paddling guide and Lonely Planet guidebook to NZ are a MUST. The Christchurch YMCA is a cheap and convenient place to stay while travelling through Christchurch.
Check out the NZ whitewater guide, available at most bookstores or kayak shops in Christchurch, at: http://www.rivers.org.nz/nzww/
There are two paddling shops in Christchurch. Talk to Rob at the Paddler Zone. Their website is: http://www.paddlerzone.co.nz/
Definitely bring your own gear and paddle. Think about renting or buying a boat down there, especially if you're planning on doing a lot of paddling. The helo creeking out of Hokitika is world class. A good source of info for buying boats is the forum at: http://www.rivers.org.nz/
Oh yeah... The first thing you do when you get to NZ is buy some sand fly repellant. Trust me.