Water was used to generate 54.9% of the electricity used in New Zealand in 2007. In 1924 hydro schemes generated less than one petajoule (a measure of energy), but by 2007 hydro generated 83.8 petajoules. Hydroelectricity is a renewable energy source with no harmful emissions into the environment, but can involve major changes to the landscape.
Hydro generation requires a reliable source of water, and a place where it can fall and drive electric turbines. Much of New Zealand is mountainous or hilly, and the rainfall is high.
The first hydro generation stations were set up by small operators such as gold dredgers, and local councils supplying lighting to towns. In 1888 the West Coast mining town of Reefton was the first to have lighting provided by hydroelectricity.
In 1896 the government passed a law that prevented people from building hydro schemes without their permission. It began building hydro stations in the early 1900s. The first major station was at Lake Coleridge, which began supplying Christchurch with electricity in 1914.
North and south grids
During the First World War government planned high-voltage transmission grids in the North and South islands. The grids would carry power from isolated hydroelectric stations to cities and towns.
In the 1920s and 1930s power stations were built in the North Island, on the Waikato River, around Lake Waikaremoana and at Mangahao in Horowhenua. In the South Island the Monowai and Waitaki stations were built.
But power generation could not keep up with demand. After the Second World War very large hydro schemes were built on the Waikato River and at Roxburgh to meet the severe electricity shortage.
A national transmission grid was set up in 1965, when a cable was laid across Cook Strait. Peak waterflow in the South Island occurs in spring and summer, whereas in the North Island it occurs in winter. This ensures year-round supply.
To supply the national grid, a series of power stations were built in the South Island. They fed the growing North Island demand for electricity.
Dams and canals
In 1965 a massive earth dam was completed on the Waitaki River at the Benmore station. A system of dams and power stations was built on Lakes Pūkaki and Tekapo. Lake Pūkaki was raised 37 metres by a dam.
In the North Island the Tongariro power scheme used a system of canals, with power stations at Tokaanu and Rangipō on the volcanic plateau. A station was also built on the Rangitāiki River.
In the 1960s people protested against a proposed dam for Lake Manapōuri which would raise the lake level. The dam was to supply electricity to an aluminium smelter.
The project went ahead, but the lake level was not raised.
People also protested against the building of a dam at Clyde on the Clutha River, which would flood a historic area in Central Otago to create a hydro lake. The government pushed the project through. It cost much more than expected because a fault line found under the dam site meant the area was unstable. The dam was finished in 1990.
Since building the Clyde dam there have been no major hydro developments – proposed schemes have been opposed by environmentalists, tourism operators, fishers and locals. But hydroelectricity remains an important part of New Zealand’s energy supply, and plans for new hydro stations or extensions to existing ones continue to be made.