Benmore dam and the Cook Strait cable

From 1896 the state held monopoly control over water for hydro development, and by 1934 it had built a large station on the Waitaki River, at the centre of the South Island. A serious power shortage existed by the onset of the Second World War and lasted into the 1950s, and blackouts were common until the early 1960s. The North Island was more populous, but had limited potential for new hydro schemes. In 1950 Bill Latta, chief engineer of the State Hydroelectric Department, proposed building more hydro capacity in the South Island and sending the power north. Te Ara notes that the two islands suited each other: river flows peaked in winter in the north and in spring and summer in the south, and the southern lakes had huge storage potential.

In 1951 the cable maker British Insulated Callender's Cables (BICC) advised that a cable under Cook Strait was possible but difficult, since there was no precedent for laying power cables in such difficult marine conditions, and in 1956 it reported that the project was 'thoroughly practicable'. In 1957 a government committee recommended a large hydro station on the Waitaki at Benmore, and approval in principle for linking the two islands' power systems. BICC laid trial lengths of cable off Ōteranga Bay in 1958, recovered them in 1960 and reported that the trial had been successful. Between 1958 and 1960 there were reservations about the risks of the cable crossing, but in March 1961 the government approved the project, with contracts of NZ£6.5 million to the Swedish company ASEA for the converter plant and NZ£2.75 million to BICC for the Cook Strait cables.

Work on the Benmore dam and power station had begun in 1958. The dam is an earth-fill structure, with a core of low-permeability clay supported by two massive shoulders of river gravel, and it is the largest earth-fill water-retaining structure in New Zealand. The Ministry of Works built the town of Otematata for the workers and their families, and many of its houses were trucked in from Roxburgh. The cables were laid from the ship Photinia in 1964. Each metre weighed about 45 kilograms, and at Ōteranga Bay, on Wellington's south-west coast, the cable floated ashore on inflated plastic pillows and took two hours to travel 800 metres from the ship. The finished link ran from Benmore on an overhead line through inland Canterbury and Marlborough to Fighting Bay, under Cook Strait by 40 kilometres of submarine cable to Ōteranga Bay, and on to Haywards in Lower Hutt, 610 kilometres in all. Converter stations at Benmore and Haywards changed the power between alternating and direct current.

The station, the dam and the high-voltage DC link to Haywards were all completed in 1965. NZHistory records that the Benmore scheme came on line that year and that electricity was sent from the South to the North Island via a new inter-island cable. The link became operational in April, and the Prime Minister, Keith Holyoake, officially opened the Benmore station on 15 May. When completed, the link was the world's longest high-voltage DC transmission scheme, with the highest power rating and the largest undersea power cables. It originally had a capacity of about 600 megawatts, and Benmore itself has a generating capacity of 540 megawatts. In 1976 the link was modified so that power could also flow south, and in 1991 new cables and new converter equipment were added. The original mercury-arc converters were finally shut down in 2012, after 47 years in service.

In Te Ara's account, the cable turned the two island networks into a single national grid. Benmore was followed by Aviemore in 1968 and by Tekapo B in 1977, which was connected by canal to the Tekapo A station. Lake Pūkaki was raised by 37 metres behind a high dam in 1977, and the Ōhau A, B and C stations were built between Lakes Ōhau and Benmore from 1979 to 1985. The upper Waitaki became a chain of stations, lakes and canals, and Lakes Tekapo and Pūkaki, with their valuable water storage, were in Te Ara's words 'fully utilised for hydro development'. Environmental and social concerns began to affect hydro projects from the 1960s, when the first sustained opposition came over Lake Manapōuri, and after 1990 the government commissioned no more large hydro stations. In 2007 the cable across Cook Strait was still carrying South Island hydro power to the North Island.