Which glacier is the largest and longest glacier in New Zealand?

The largest and longest glacier in New Zealand is the Tasman Glacier.

Officially known as Haupapa / Tasman Glacier, it lies on the South Island within Aoraki / Mount Cook National Park. The glacier descends from the Southern Alps toward the Mackenzie Basin and covers about 101 square kilometres, making it New Zealand's largest glacier.

At roughly 23.5 kilometres long, it is also the country's longest. The ice can reach about 4 kilometres in width and 600 metres in thickness. Its lower end now terminates in Tasman Lake, a proglacial lake formed as the glacier retreated and its terminal ice melted back.

Tasman Glacier is sometimes confused with Franz Josef Glacier or Fox Glacier because those West Coast glaciers are more accessible and widely photographed. They are not larger or longer. Tasman Glacier remained close to 28 kilometres long through much of the twentieth century, but rapid retreat accelerated from the 1990s. Between 2000 and 2008 alone, its terminus receded about 3.7 kilometres, accompanied by major growth of Tasman Lake.