The Poutu Canal is a man-structure that carries water taken from the Poutu Dam on the Tongariro River under SH1 and SH46, delivering it to the Poutu Stream and into Lake Rotoaira, in order to then pass through tunnels and down to the 240MW Tokaanu Power Station, part of the Tongariro Power Scheme.
The canal was constructed to interfere with as few natural streams and watercourses as possible and this location gives a fine example of this, where the canal passes over, high above, the Mangahouhounui Stream. You could call this a flume or an aqueduct, or even a canal bridge, but I reckon aqueduct sounds the best.
The Mangahouhounui Stream that you can glimpse way down in the ravine below you originates on the north-western slopes of Mt Tongariro, and merges with the Poutu Stream before flowing into the Tongariro River.
Walking and cycling access is permitted alongside the Poutu Canal, but do watch out for the edge and take care of little ones - the canal is very deep and can be fast flowing when water is being diverted. There are escape ladders located every now and then along the sloped canal walls. Fishing is not allowed in the canal.
You are looking for a 400ml Sistema container with log sheet and swaps, located in an obvious spot. Please don't drop it over the "edge".
BYOP. Good luck and happy caching!