Fiordland National Park was established in 1952, though land that later became part of the park was set aside as early as 1904. It is New Zealand’s largest national park covering a vast and remote wilderness area of 1.2 million hectares, making it one of the largest national parks in the world.

Altitudes are highest in the northern area of the park; the highest peak is within the Darran Mountain range with Mt Tūtoko at 2746m. The underlying rock of the park is mostly granite, gneiss and diorite. These igneous rocks are among the oldest in New Zealand (mostly Ordovician) and are also among the hardest and most resistant to erosion. Thus they have remained almost unchanged since latest ice ages, when an ice cap was covering the mountain areas of Fiordland.

The glacier tongues that flowed from it gouged the 14 fiords that fringe the southwest seaward edge of Fiordland National Park. Shaped like massive knife cuts into the land these fiords were carved during successive ice ages and were 100,000 years in the making, the final details added during the most recent ice age just 10,000 years ago. On all sides of the fiords, spectacular waterfalls tumble incessantly as the region's plentiful rainfall finds its way to the sea.

Fiordland is subjected to very high annual rainfall due to the prevailing westerly weather pattern which is characteristic of the west coast of the South Island. Milford Sound received an annual average of 6526mm annually over the period 1969-1998, spread over 180 days of rain per year. Temperatures are mild, at least at low altitude: at Milford Sound, over the same period, mean maximum temperature in summer (January) was 18.8 C, against mean minimum temperature of 1.3 C in winter (July).