Hi are you doing a particular track, or are you just venturing into the area for a look see? As far as I know from a friend who I tramped with, there are a few routes in the area that cover more than one of the rivers in the park. I know that the Roaring Lion is frequented by whitewater rafters along with other tributaries.
One track I know of is the Leslie-Karamea Track. It is approximately 90km long one way, takes around 5-7 days, highest point around 2000m and has several DOC (Department of Conservation) huts. It can be accessed from a number of points.
Anther is the Wangapeka Track, but I think this is only a short track and can be incorporated into the main track I have mentioned.
You could probably get some more info from
http://www.doc.govt.nz but the best track description I have found is at
http://www.backcountry.com/new-zealand- ... oute-guide. I reccomend you start here. If you can get hold of the nzWilderness magazine october issue there is an article on track guides for the Kahurangi National Park. Not sure if this will give you much, but you never know.
Looks like a great adventure, hope this helps, good luck.