Hi, Alastair, welcome!
Gorgeous scenery--we're all jealous! You've reminded me that I must get down to NZ.
Before reading the text below I came to the conclusion that you'd used a hard grad, as (I think) I can see the line. Might be that that's how the rocks look, but I suspect I'm seeing the GND's hard filter edge.
With such gorgeous light on the clouds, it's a shame the highlights are still blown. I do like the shutter speed you've chosen for blurring the water motion, so it would be nice to see a little more filtration on the sky, to hold those highlights.
Compositionally it would be nice to give the elements of your scene more room to breathe. Specifically, the large rock on the right being cropped so severely creates a fair bit of edge tension, which to me, feels somewhat at odds with the tranquility of the water and light of the scene.
None of this is advice, really, as everyone visualizes a scene differently; it's better to say that these are some of the things that I would be exploring in my own work.
Thank you for sharing, Alastair,