Lake Pukaki is the largest of three roughly parallel alpine lakes that run north to south along the northern edge of the Mackenzie Basin in New Zealand's South Island. The other two are Lake Tekapo and Lake Ohau. All three were formed the same way: as glaciers retreated at the end of the last ice age, their terminal moraines, the walls of debris bulldozed ahead of the ice, blocked the valleys behind them, creating natural dams. Pukaki covers 178.7 square kilometres, with a surface elevation normally ranging around 518 metres above sea level. The Tasman Glacier, New Zealand's longest, feeds the lake from its northern end, and on clear days Aoraki / Mount Cook is visible at the head of the valley, its snow-capped summit framed perfectly above the turquoise water.

Between 1940 and 1967, every five pound note in New Zealand carried an engraved image of Te Kohai Island sitting in Lake Pukaki with the Southern Alps rising behind it. The scene became so familiar that the island earned the nickname "Five Pound Note Island," and in 1964 Evelyn Hosken titled her memoir about life in the Pukaki area Life on a Five Pound Note. The engraving was based on a photograph taken in autumn 1934 by Ashburton photographer Bobbie Barwell, who had passed the image to her local MP, Horace Herring, thinking it might be useful for publicity. She was surprised to find the top half of her photograph engraved on the national currency. The island is gone now. The lake was raised twice, by 9 metres in 1952 and 37 metres in 1976, to increase hydroelectric storage capacity. Te Kohai Island vanished beneath the rising water, a piece of national iconography drowned by industrial need.

Lake Pukaki is no longer simply a lake. It is a reservoir, the centerpiece of the Waitaki hydroelectric scheme that supplies a significant share of New Zealand's electricity. The original outflow at the lake's southern end has been dammed, and canals now carry water through the Ohau A power station to Lake Ruataniwha. Water from Lake Tekapo is diverted through a canal to the Tekapo B power station on Pukaki's eastern shore, feeding the system from a second source. The operating range of the lake is 13.8 metres, the band within which it can be artificially raised or lowered, giving it an energy storage capacity of 1,595 gigawatt-hours. Combined with Lake Tekapo's 770 gigawatt-hours, the two lakes provide over half of New Zealand's total hydroelectric storage capacity. In 2012, Environment Canterbury approved a change allowing Meridian Energy to lower the lake an additional 5 metres below its minimum level of 518 metres in the event of an energy crisis.

In August 2020, a scrub fire erupted on the shores of Lake Pukaki and burned for twelve days. The blaze consumed 3,500 hectares and required a firefighting force of 14 helicopter monsoon buckets, two fixed-wing aircraft, and 10 fire engines before it was extinguished at a cost of one million dollars. Both State Highway 8 and State Highway 80 were closed, and Aoraki / Mount Cook Village was evacuated. Environmentalists pointed to the spread of wilding pines in the Mackenzie District, combined with dry conditions, as fuel for the fire's intensity. The nearby Pukaki Scientific Reserve, a 32-hectare patch on the lake's western shore, narrowly survived. It is home to the nationally endangered moth Izatha psychra, a species found nowhere else. On the southern shore, Ngai Tahu once maintained a seasonal food-gathering settlement called Punatahu, a reminder that people have lived alongside this water for centuries, adapting to its moods long before anyone thought to dam it.