I didn’t have time to walk any further up than the Dredge Burn, but from here it’s a clear PR2/Class II-/I+ run out, no doubt all the way to the lake, although I got out at the Route Burn, at which point it’s really Class I. The Dart is full of glacial silt which makes it very opaque, and it’s a big swift river, I had to pay attention in the upper part where there are some obstacles and small wave trains to negotiate. The first section (pictured) was the most fun and scenic, after about the Beans Burn the valley really opens out and you’re into braided gravel flats - stay in the main channel, otherwise you’ll be scraping across shingle.
Jet boats operate on the river (http://www.dartriver.co.nz/), but no further up than Sandy Bluff (and not always that far up, depending on the river). You can phone them and they will make a note of when you’ll expect to be on the water, so the boats can watch out for you (they can have 3 - 4 running at once). You’ll hear them coming a mile off though. Packrafts really seem awfully elegant in comparison.
If you’re considering doing the Dart-Rees loop and are wondering which way to go around: the Dart side definitely lets you cover more ground quickly + easily than the smaller Rees river, which is too small upstream of Hunter Creek, often too shallow even downstream of here, and has a section of rocky rapids near the carpark. On the other hand the Rees is boat-free and has clear water.
I’m sure there’s more raftable water higher up the valley - to be continued !