The Roxburgh Dam is the earliest of the large hydroelectric projects in the southern South Island of New Zealand. It lies across the Clutha River / Mata-Au, some 160 kilometres (99 mi) from Dunedin, some 9 kilometres (5.6 mi) to the north of the town of Roxburgh. The settlement of Lake Roxburgh Village is close to the western edge of the dam.
With power cuts being applied across the South lsland by June 1956 the Minister of Works requested the contractors to concentrate all resources on work that would bring forward lake filling as far as possible. To encourage to the workforce the government offered a bonus of £2 per week plus £1 per day that the lake was filled before 19 August. At midnight on the 21 July 1956 lake filling began and the lake level commenced rising at an average of 3 ft an hour.
As the lake began to fill increasing levels of water began to flow from the drainage channels behind the grout curtain in the right abutment, which indicated that the grout curtain was faulty. Investigations concluded that further grouting would have to be performed (which took about a fortnight) before the lake could be raised it its final level. The decision was made to allow the lake to fill to no further than the crest of the spillway while the contractors began drilling and inserting more grout.
By 11:20 am on 23 July 1956 the lake had filled to the crest of the spillway water. With a desperate shortage of electricity effecting the South Island, commissioning of the No 1 machine immediately commenced. Once the engineers were satisfied that the machine was fit for service it was connected at 6 pm to the national grid. Due to the reduced head the machine’s output was limited to 30 MW. By the end of the next day No.2 machine had completed commissioning and was also connected to the system. This allowed the 220 kV line to Islington to be bought into service as two machines were needed to provide sufficient reactive power to charge the long length of line. The third machine was commissioned on 18 August 1956 and the fourth machine on 11 December 1956. The power station was officially opened on 3 November 1956 by Stan Goosman.
Delivery of machines 5 to 8 began late in 1959.
The commissioning of Roxburgh removed the need for power restrictions in the South Island and ensure a surplus of power for many years.
To connect the new power station to the major load centres, a 52-mile-long (84 km) new 110 kV wood pole line was first built to Gore. The linemen then commenced constructing a 89-mile-long (143 km) double circuit 110 kV overhead transmission line using lattice steel towers to the Halfway Bush substation at Dunedin which was completed in July 1955 at a cost of approximately £500,000.
The principal connection, however, was a new 266-mile-long (428 km) 220 kV single circuit overhead transmission line built using lattice steel towers from Roxburgh to a new substation at Islington on the outskirts of Christchurch. By 1949 the surveys for this line were well under way with by 1951 the construction camps established and the material was on order. By 1954 the first section of the line had been completed, which allowed it to carry power from Tekapo A to Christchurch. A second section as far south as the Waitaki Valley helped improve supply conditions during winter. The Roxburgh-Islington line cost approximately £1,000,000 and was completed by the winter of 1956.
The power station consists of an 1,170-foot-long (360 m), 185-foot-high (56 m) concrete gravity dam from which eight steel penstocks supply water to a powerhouse containing the turbines. The penstocks change from an 18-foot-square (5.5 m) intake section to 18 ft in diameter before tapering to 15 ft (1.4 m) where they enter the scroll case. Spillway gates are located on the West (right) side of the dam. The designers anticipated a 500-year flood of 120,000 cusecs (3398 m3/s). As a result, the spillway was designed with a capacity of 150,000 cusecs (4247 m3/s).
At the base of the spillway are sluice gates designed to pass 80,000 cusecs (2265 m3/s). During construction these sluice gates were used to divert the river via a diversion channel. The upstream section of the diversion channel was unlined and followed an old natural channel of the river before reaching the spillway and sluice gate block which is curved at the exit to direct water away from the outdoor switchyard. The surfaces were finished to a high standard to ensure a smooth flow of the water during medium and high flows.
The superstructure of the powerhouse is constructed of welded steel framed clad in precast concrete panels.