Hi Gav.
After looking at the map,
it is my conclusion that the name, "Sluice Box", refers to the restricted area thats between the two spurs that seem to be squeezing in upon the "Maruia" river at this point,forming a possible "Gorge" there,which is also forming a large natural sluice.
That statement doesn`t mean to say that,thats the correct interpretation!
Cheers---Colin.