In each of the North and South Islands the network of power stations, transmission lines, and substations function as one system. Control offices at Whakamaru and Islington keep in constant touch with generating stations and substations so that the fluctuating demand for power may be met at all hours.

The South Island system started in 1915 when 63 miles of transmission line linked Coleridge power station with Addington substation. When Waitaki came in in 1935, the network of lines brought the Dunedin City's generating station at Waipori, first operated in 1907, into the system. Subsequently, Southland, the West Coast, and Nelson-Marlborough were incorporated into the South Island grid, where power is conveyed at 33,000, 66,000, 110,000, and 220,000 V. Higher voltages have been used as new power stations have been built and power conveyed over longer distances.

North Island development of hydro-electric resources began in 1919 with the purchase from the Waihi Gold Mining Co. of the Horahora power station and its transmission system. (Horahora is now submerged in Karapiro Lake.) The system extended in the Waikato area and by 1925 included Auckland. In 1925 power was carried from Mangahao power station to Wellington, and this system reached Napier in 1927. In 1929 it was linked to Tuai, the first Waikaremoana station, and two years later New Plymouth was connected to the network in the southern portion of the North Island. Meanwhile, a 110,000-V line was being built between Stratford and Arapuni, which linked the two North Island systems in 1934. The map shows the present network of 50,000, 110,000, and 220,000-V lines which transmit power in the North Island.

As in the South Island, the network has been extended when new stations have been commissioned. In 1965 the North and South Island networks were at length interconnected by means of the Cook Strait cable.