There are a number of different route options for this trip but because it is only suitable for experienced trampers I will leave it over to you to do determine the best route for your party. Whichever way you go there is a significant portion that is off trail.
Just in case anyone is interested the way I did it was to park the car at the Kaniwhaniwha track head then walk up the Bell track to the Dumb Old Tree (GC45HEH). Dropped down to the stream and followed it up to the waterfall then made my way up the steep hillside NW of the falls and straight line from there to the Bell Track. Down the Bell Track and back to the carpark.
About 20 km all up and it took me over 10 hours. My work colleague said he did it in about 7 hours but he is fit. It sounds like his route was similar but he did the loop in the reverse direction i.e. stream section last.
Have fun!
WARNING: This trip shouldn’t be taken lightly. Please ensure that you are adequately prepared for the journey. There is no cell phone coverage in this remote area so don’t go alone. A 1:50,000 topomap and compass are essential backups to the GPSr which is bound to crap out at the worst possible moment. Take a bearing before you leave the track or stream. Don’t forget to tell someone where you are going and when you expect to be back, take lots snacks (plenty of good water most of the way) and dress appropriately. Depending on your route the natural hazards will include cold water, slippery boulders, steep terrain and the usual joys of off track travel in the NZ bush such as supple jack, cuttygrass and bush lawyer!