NZ can be a little tough to break into. It's better to go earlier so you can complete the rafting association swiftwater course and qualify for class IV / V guiding jobs by checking out with an official guide instructor.
Around Rotorua you're pretty much guiding the Kaituna with an occasional weekend release on the Wairoa (you'd pretty much just train if you weren't there more than 6 months) and occasional trips on the Rangitaiki. There was a gluttony of guides when I was there in 2005-06.
There are also companies out of Queenstown on the Shotover and Dart (more snow melt dependant) and my favorite place and where I would try and work although it's tough to get a job... New Zealand Adventure New Zealand Adventure Lodges Rangitikei
Let me know and I'll send you a reference if you get serious about going.
You qualify for one work visa for one year anytime before you turn 30 in NZ and Aus so you should get on top of that.
I've never been to Africa, but I would guess there's a lot more opportunity to see a lot more rivers in NZ. Africa would probably be far superior for a cultural experience and might actually offer as good or better an opportunity for work...
Maybe try and work out of Cairns, Australia on the Tully where I hear it's easy to find work then take a month or two to go to NZ and paddle.
All of it sounds pretty great to me right now