The spit runs west to east from Cape Farewell, with about 25 kilometres of stable dunes and another 5 kilometres of mobile sand that shifts with the weather. On the northern, ocean-facing side, crescent-shaped dunes called barchans migrate eastward under prevailing westerly winds, their steep leeward faces carving new shapes with every storm. The southern side, sheltered by the spit itself, is calmer - vegetated, lake-dotted, and fringed by tidal flats that extend up to 7 kilometres at low tide, exposing 80 square kilometres of mudflat. The total area covers 11,388 hectares, but only about 1,961 hectares rise above mean high water. The rest is intertidal: a shifting boundary between land and sea that redraws itself twice a day.

In summer, roughly 29,000 shorebirds crowd the spit - more than 10 percent of New Zealand's entire shorebird population. Bar-tailed godwits arrive in their thousands, having flown nonstop from Alaska in the longest known migration of any land bird. Nearly 12,000 godwits gather here in summer, representing 9 percent of the species' total flyway population. South Island pied oystercatchers number close to 7,000. At the spit's eastern tip, Australasian gannets nest in a colony that has exploded from 75 nests in 1983 to 10,000 birds by 2022 - the only sea-level gannet colony in the world, breeding just metres above the tide on sand rather than the high rock formations gannets normally prefer. Black swans moult here in numbers that account for up to 15 percent of the national population. Farewell Spit was designated a Ramsar Wetland of International Importance in 1976, and in 2014, BirdLife International recognized it as an Important Bird and Biodiversity Area.

The same gently sloping beaches that make Farewell Spit a paradise for wading birds make it lethal for whales. Long-finned pilot whales navigate by echolocation, and the spit's gradual incline returns confusing echoes, drawing pods into shallower and shallower water until they strand. The phenomenon has been documented since 1867, and mass strandings occur with grim regularity - herds of dozens, sometimes hundreds, beaching themselves on the golden sand. Volunteers arrive to keep the whales wet, pouring water over their skin to prevent overheating, but refloating a beached pilot whale is physically exhausting and often futile. Many of the whales re-strand after being guided back to deeper water, as though the spit's acoustic trap still holds them. Scientists call it a whale trap. The term is clinical, but the sight of it is not.

Ships met the spit before whales did. In the age of sail, the shoals and currents around Farewell Spit wrecked vessel after vessel, and in 1870 a timber lighthouse was lit at the eastern tip to warn mariners. The salt-laden winds destroyed the hardwood tower within three decades, and in 1897 it was replaced with the steel lattice structure that still stands - the only lighthouse of its kind in New Zealand. Its 27-metre tower throws a beam visible for 35 kilometres, taller than most New Zealand lighthouses because its foundations sit barely above sea level. The last keeper left in 1984 when the light was automated. Today the lighthouse stands alone at the end of the spit, reachable only by permit-guided tour, with 10,000 gannets nesting a 30-minute walk beyond it.

Farewell Spit was leased for grazing from the 1850s, and cattle, fire, and introduced predators ravaged the vegetation and wildlife. Protection came in 1938, though wild cattle lingered until 258 were removed in the 1970s. The spit became a nature reserve in 1980, and public access was restricted to the first 4 kilometres. In 2025, construction began on a 3.5-kilometre predator-proof fence at Wharariki Beach, with pest elimination targeting pigs, possums, stoats, and hares working eastward from the spit's tip. The goal is ambitious: a predator-free Farewell Spit by the end of summer 2026. If it succeeds, the heaped-up sand will belong entirely to the birds again.