When I went to Hamner Springs, many moons ago, I remember there is(or was) a picnic area with a stream that was real easy to get at.Just up the road from hotpools.
From my experiance at searching for koura earlier this year, it is a likely spot.
Sorry I cant be more descriptive but I do think that if you go buy yourself a butterfly net and a decent pair of gumboots(or if feeling like a tuff guy, just go barefoot in the freezing waters) and place net downstream of rocks and lift the rocks.
Persistance pays.
It took me about an hour of searching till i found mine, then once I sussed it out they were found everywhere.
You want a stream where the flow is not breaking the surface otherwise its impossible to see anything.
http://www.fnzas.org.nz/fishroom/native ... 30902.html
Thats a link to my koura hunt thread.
HTH