All but the first 300m of the gorge can be seen from the helicopter; be sure to fly over the gorge and pay attention. The rapids are segmented, and vaguely recognizable from the air. What you can't see (the first 300m) is very committing, tight vertical walls, but manageable, pushy class III white water exists here. Most of this you can see from the swing bridge at the entrance.
Once through the committing section, the walls open up slightly and there is room to get out on the bank/in the river bed to scout/portage. Rapids also get more serious from here down. Some must be paddled, some must be portaged. The atmosphere is fairly intense. Everything seems enclosed, and the water seems confined and powerful.
The rapid at the exit of the gorge is a good indicator of the style of kayaking within the gorge. This rapid is accessible at the end of the Morgan Gorge portage.
The river bed seems somewhat mobile, and there have been differing descriptions of rapids over time. Treat every descent as a first.
Check out video by Dave Kwant http://www.kayaksession.com/wild-and-hidden-places-a-kiwi-classic-kayak-...