The Wairoa River

 (that's me getting decapitated by the other guy's paddle.  No worries, he was a bit preoccupied at the moment.)

 
The Wairoa river is only available 26 times per year, when the Ruahihi Power Station opens its gates.
It starts with a few warm-up rapids, concluding with two Grade 5 runs, The Waterfall and the Rollercoaster
 
 
Mothers Nightmare
Waterfall
Roller Coaster
Last Falls
 
Here's the rafting company:

They ran such an excellent trip that I went back the very next day for this.

 
This segment is the locale of the annual Wairoa Whitewater Extreme Race:

(Look for the Other Stuff-Extreme Race link for the most recent event)

 
The NZ Freestyle Kayak Association has a good narrative of the river.
 
 

The NZ Recreational Canoeing Association, with excellent resources on NZ rivers:

 
Here's a map of the course of the river.