Tongariro Power Scheme

The Tongariro Power Scheme is a 360 MW hydroelectricity scheme in the central North Island of New Zealand. The scheme diverts water from tributaries of the Rangitīkei, Whangaehu, Whanganui, and Tongariro rivers. These rivers drain a 2,600-square-kilometre (1,000 sq mi) area including Ruapehu, Ngauruhoe, Tongariro and the western Kaimanawa Ranges. The water diverted from these rivers is sent through canals and tunnels to generate electricity at three hydro power stations, Rangipō (120 MW), Tokaanu (240 MW) and Mangaio (2 MW). The water is then discharged into Lake Taupō where it adds to the water storage in the lake and enables additional electricity generation in the succession of power stations down the Waikato River.

The scheme generates approximately 1350 GWh of electricity annually, and contributes 4 percent of New Zealand's electricity generation. It is currently operated by electricity generation company Genesis Energy.

From its inception to the current day tensions have existed between the need to generate electricity and cultural and environmental issues.

Plans to realise the potential of electricity generation in the central North Island volcanic plateau date back to the early years of the 20th century. A Californian engineer, L.M. Hancock, visited New Zealand in 1903 and, with engineer P.S. Hay, surveyed localities where power could be generated. In 1904 Hay's report to the government New Zealand Water-Powers identified a number of potential sites around the central plateau. These included Lake Rotoaira, the Whakapapa River and the Whangaehu River. A dam could be built on the Poutu River feeding into Lake Rotoaira and the amount of power generated could be increased by adding water from Tongariro or Ngauruhoe. While he identified the generation potential of the Whangaehu River, Hay noted that it was polluted by volcanic acids and chemicals making it unsuitable. Between the 1920s and 1950s a number of investigations on the potential of the volcanic plateau were undertaken by engineers, mostly in the Public Works Department. In 1955 the British consulting engineering company Sir Alexander Gibb & Partners was contracted to conduct preliminary investigations; they produced a preliminary report in 1957. The government authorised the scheme in 1958 through an Order in Council which allowed water to be taken from five catchments without giving notice or gaining consents and approved the scheme in 1964. Gibb and Partners conducted further field studies and more detailed design which culminated in their report in 1962 which recommended the construction of a power scheme in five stages:

The final project differed from Gibb's design: only one dam was built at Moawhango, the Kaimanawa station was not built, and an additional power station was built at Mangaio. The four stage project commenced in 1964 and was completed in 1983: the Western Diversion from 1964 to 1971, the Tokaanu project (Rotoaira Diversion) from 1966 to 1973, the Eastern Diversion from 1969 to 1979, and the Rangipo project (Tongariro Diversion) from 1974 to 1983.

The scheme was built by the Ministry of Works and the project engineers were Warren Gibson and Bert Dekker. The New Zealand Electricity Department (NZED) operated the generation and transmission of electricity. NZED became part of the Ministry of Energy before becoming part of the Electricity Corporation of New Zealand (ECNZ) in 1987. The government transferred all generation and transmission to ECNZ in 1990. In 1999 with the reform of the energy sector and break-up of ECNZ the Tongariro scheme was transferred to Genesis Energy.

The Western Diversion was the first part of the Scheme to be constructed between 1964 and 1971. It starts at the Whakapapa River, taking water into a 16.5-kilometre (10.3 mi) long tunnel to Lake Te Whaiau; intake structures collect water from the Ōkupata, Taurewa, Tāwhitikuri, and Mangatepōpō streams along the way. The Te Whaiau dam is an earth-fill dam 13.7 metres (45 ft) high and 54.8 metres (180 ft) metres long.

Water from the Whanganui River is diverted via a short tunnel into the Te Whaiau Stream, which joins the water from the Whakapapa and other intakes, all flowing into the Te Whaiau dam. Both tunnels, begun in 1967, were constructed by the Italian construction company Codelfa-Cogefa which specialised in tunnelling. Water from Lake Te Whaiau flows into the Ōtamangākau Canal which feeds Lake Otamangakau, where water also flows from the Ōtamangākau Stream. Lake Ōtamangākau is formed by an earth-fill dam 20 metres (66 ft) high and 259.9 metres (853 ft) long.