Horahora was the first power station on the Waikato River and was built to provide power to the Waihi Gold mines. This began to operate in 1914 with a 50 kV power supply line stretching over 80 km from the plant.
The power station was submerged beneath Lake Karapiro in 1947.
Here is the general area from LINZ aerial footage.
The power station was in the area towards upper right where there is an island in the lake.
The old NZMS1 map of the area. It's possible the road used to run a bit to the west of where it is now.
Internet sourced photo showing the east abutment of the power station dam with the steps leading down to the submerged powerhouse. This photo came from the blog posting linked below.
The power station and supply line cost around 212,000 pounds at the time, which in today's money is about $32 million. As it happens the mine had already peaked by that stage so the full capacity of the station was never used by the mine. The Government took over the station in 1919 but did not actually pay for it until 1934 at the original price, which was a rather generous scheme as by 1934, the original price had appreciated to 277,000 pounds. In 1924 the station was increased in size and transmission lines were built to Auckland. The copper and other metals from the generators was salvaged from the depths of the lake in 1971.