Hmmm. There are some minor riffles, like these:
And lots of boringly gorgeous water like this:
(The wild stretch is the Waikato River just below Lake Taupo and the fall is called Hukanui. I think they banned boating on that reach. The tranquil stretch is on the Waioeka, above Tauranga.)
There's a big volcanic cone near New Plymouth called Taranaki, 2518m. Lots of streams run off it to the sea. It's not far to the Whanganui River, which is a classic NZ float trip through amazing gorges: check out the NZ film River Queen
that was shot there.
Summed up, there's a lot of good boating water on North Island.
For a listing, go to http://www.rivers.org.nz
and click on the River Guide. Check out the areas on the list from Waikato to Wellington.