Future plans are affected by the fact that hydroelectric potential is mainly in the South Island and the main population is in the North. Further, maximum run-offs of water occur at different times – in the North Island, usually in winter; in the South, in the summer, when the snow melts. The Combined Committee in 1958 recommended that approval in principle be given to interconnecting the North and South Islands by a direct-current submarine cable designed for a capacity of 600,000 kW and capable of extension. The Government adopted the report and in 1958 a trial length of cable was laid in Cook Strait. The recommendation was opposed on technical grounds and in November 1958 the Government asked an English firm of consulting engineers to report on the scheme. The comprehensive report recommended in essence that power from Benmore on the Waitaki River should be transmitted to Haywards, near Wellington, by direct current at 500,000 V and that it should cross Cook Strait by means of submarine cables from Fighting Bay, north-east of Blenheim, to Oteranga Bay, near Cape Terawhiti.

Extensive and careful investigations having established that the difficulties of installing a power cable across Cook Strait could be overcome, the Government (in March 1961) approved the scheme for connecting the North and South electric power systems. The route selected was the shortest and avoided the problem of magnetic interference with ships' compasses in the approaches to Wellington Harbour. Three cables some 1,000 yards apart were laid, one of which is a spare. The cable is a pre-impregnated, high-pressure, gas-filled type with a diameter of 5 in. and weighing 90 lb a yard. The £2 million contract for the manufacture, laying, and terminating these three 25½-mile-long submarine cables was placed in 1961.

Power for the North Island is generated at Benmore, which when completed will produce 540,000 kW from six machines. First power was produced in January 1965. This scheme is probably New Zealand's largest single undertaking. The 2,700-ft-long dam gives a 305-ft head of water and has created a 30-sq.-mile lake. A building as large as the powerhouse houses equipment necessary for converting the generated energy to direct current; and at Haywards substation, Wellington, similar equipment has been installed for reconverting the power to alternating current. The £6 million contract for this converter equipment was let in 1961. In 1962 the Electricity Department started to build the 354-mile, 500,000-V direct-current transmission line between Benmore and Haywards and by January 1965 the line was completed. Some sections pass over high, rugged country.