The popular Kepler Track follows the Iris Burn out to the east end of Lake Manapouri, close to Te Anau.
I gave this a go yesterday on the way out from a walking trip: it was a bit frustrating since there just wasn’t enough water, and the river didn’t seem to get any bigger towards the lake. With higher volume it could be a fun (but still small/narrow/fiddly) III going down to II closer to the lake I think? Didn’t spot anything harder or particularly steep (I couldn’t see the whole river though). Would be interested if anyone else has tried this with more water, access is nice and easy with the groomed Kepler track running up beside the river. Alternatively you can come over the tops for the view, although the river seems too small until at least after the slip.
It has been hot + dry recently - yes even in Fiordland - so maybe I will revisit after some rain.