I was thinking you'd have a month or two. Get the guidebook for sure.
If you've only got a couple days, where you boat is pretty dependent on where in the country you are. In late August, there should be plenty of water in most places, but rainy/cold spells are common. North Island east coast has decent boating weather then, but most of the 'yak rental outfits pack up for winter, so it can be a challenge to hire a boat.
But there's some gorgeous water. Here's Huka Falls, on the Waikato River below Lake Taupo: typical Kiwi class IV.
Between the commercial timbering, farming and livestock, many North Island rivers are in sad shape. But there are a few still under native bush that are gorgeous: below is the tranquil part of the Waioeko, which reaches the east coast of N. Island at Opotiki. Upstream are bedrock channels and some sporty rapids: a nice 2-3 days of boating. Tree ferns foreground. Clean gravel on the bars and the water is like Absolut with a dash of Midori.
Both photos were taken in August, 2006.
If you'll be in Auckland, it might make sense to try sea kayaking instead. There's a rental place on Tamaki Drive, by the Canoe Club.
Have a good trip—