Lake Manapouri’s hydroelectric potential had long been recognized, but nothing was done until the 1950s. Consolidated Zinc Pty of Australia wanted to smelt their Queensland bauxite into aluminium – a power-hungry process – as cheaply as possible, and alighted upon Lake Manapouri. They approached the New Zealand government, who agreed to build a power station on the lake, at taxpayers’ expense, while Rio Tinto’s subsidiary Comalco built a smelter at Tiwai Point, near Bluff, 170km to the southeast.
The scheme entailed blocking the lake’s natural outlet into the Lower Waiau River and chiselling out a vast powerhouse 200m underground beside Lake Manapouri’s West Arm, where the flow would be diverted down a 10km tailrace tunnel to Deep Cove on Doubtful Sound. By the time the fledgling environmental movement had rallied its supporters, the scheme was well under way, but the government underestimated the anger that would be unleashed by its secondary plan to boost water storage and power production by raising the water level in the lake by more than 8m. The threat to the natural beauty of the lake sparked nationwide protests, though the 265,000-signature petition prepared by the “Save Manapouri Campaign” failed to change the government’s mind. It was only after the 1972 elections, when Labour unseated the National Party, that the policy was changed and the lake saved. The full saga is recounted in Neville Peat’s Manapouri Saved.
The Manapouri Underground Power Station took eight years to build. Completed in 1971, it remains one of the most ambitious projects ever carried out in New Zealand. Eighty percent of its output goes straight to the smelter – which consumes around fifteen percent of all the electricity used in the country, and it is widely perceived to be an unnecessary drain on the country’s resources. Because of the unexpectedly high friction in the original tailrace tunnel the power station had always run below 85-percent capacity, and to boost power production a second parallel tailrace tunnel was built in the late 1990s.