This is the Poutu Dam, at the downstream end of the Poutu Canal. It's a kind of interesting structure, because from here on into Lake Rotoaira, this waterway used to be the outflow of the lake, into the Poutu Stream that then flowed eastwards and into the Tongariro River. But now, the canal pumps a steady flow of water into Lake Rotoaira instead - this water then makes its way across the lake and into the intake for the Tokaanu Powerstation.
The Poutu Stream, however, is still fed a constant flow of water out of the dam, basically turning 180 degrees from the water coming in from the canal. If you look down to the stream you can see how deep the channel is from the dam heading towards the lake. In times of high flow there is a fairly spectacular fountain of water coming out from the base of the dam into the stream, but when flow is light, there is a smaller pipe on the side that lets water out into the stream - you can see this from GZ.
You are looking for a cache custom designed to fit into its hiding spot - it should not be hard to find. BYOP.
Good luck and happy caching!