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Yep, done the Wanganui a number of times in high flows, where whirlpools are formed at the foot of rapids on a bend. The centre of the whirlpool can be a meter lower than the surrounding water level....

Good point re our topography.

I think I am right in saying that nowhere in NZ is more than 100km (62 miles) from the sea. Yet the mountains can go to 12000 ft in that distance.

So many rivers...

You'll find our rivers are full of rocks, with some water in them and can be quite tight, whereas most of your rivers are full of water with some rocks in them.

Youtube some of the named rivers...