Whakamaru Power Station is a hydroelectric power station on the Waikato River, in the North Island of New Zealand. It is the fourth hydroelectric power station on the Waikato River. Lake Whakamaru is one of the larger hydro reservoirs on the Waikato river. The power station is owned and operated by Mercury Energy. The adjacent Whakamaru switching station is operated by Transpower and is one of eight reference nodes on the New Zealand national grid. History Development work started in 1949 with the construction on of road from Mangakino which at the time was the operational centre of the Waikato hydroelectric scheme. Firstly a diversion channel long, deep and wide was built taking 3 years to complete. The foundation rock turned out to be deeply cracked and filled with clays rendering it partially porous. Shafts were sunk into the rock and this allowed the clay to be cleared and cement grout was later forced into the rock which was then back-filled with concrete. Transmission Whakamaru is a major node in the North Island electricity grid. Electricity generated by Whakamaru and the other five Waikato hydroelectric power stations, and the geothermal power stations in the area, converges at the Whakamaru substation on the western side of the dam. It is transmitted from there via 220 kV lines to the Bay of Plenty, Hawke's Bay, Manawatu and Hamilton. The largest transmission route is to New Zealand's largest city, Auckland, approximately to the north – three 220 kV circuits connect Whakamaru to Otahuhu in southern Auckland, with an additional circuit serving Otahuhu via Hamilton. The main control centre for the electricity generation and transmission system for the entire North Island was established at Whakamaru in 1964. The facility continued to operate as the North Island System Control Centre until a replacement facility was established at Hamilton in 1992. In 2010–2012 the Whakamaru to Brownhill Road transmission line, a major line, was constructed to Brownhill Road, close to the south Auckland urban boundary. This reinforced supply lines to Auckland and allowed for the large amount of new geothermal generation that was expected in the Taupo region over the following 5–7 years. A separate switching station, designated Whakamaru North, was built approximately from the current substation as the terminal station for the new line, with short tie-lines connecting to the existing substation. Refurbishment In 2013 a refurbishment project was begun to refurbish the turbines and