This place reminds me of Apollos Pool, but much easier to
get to. Just follow the Zion Hill track to where is meets the
Waihuna Stream, then follow the stream through the bush until
you're out in the open.
There is then just a small waterfall (with 1.5m deep pool below) to
climb down and large fallen tree to navigate before the main
waterfall itself (or you can bush bash around on the west side to
avoid).
Soak in the last pool while taking in the view, and watching
those walking the track below through the valley.
The cache is a 200ml sistema painted black. There's plenty of
room at the waterfall edge, as the lip falls away gradually - but
don't take this for granted and always take care.
(For this trip, you might like to work out Your Number, if you haven't done so
already 
)