The Whanganui rises on the northern slopes of Mount Tongariro, one of the North Island's three active volcanoes, near Lake Rotoaira. From there it flows 290 kilometres through some of the most rugged country on the island, turning northwest before swinging southwest at Taumarunui, threading through the bush-clad hills of the King Country, and finally reaching the Tasman Sea at the city of Whanganui. Maori legend offers two origin stories. In one, Mount Taranaki's departure from the central plateau tore the earth open, and water filled the rift. In another, after the demigod Maui caught the great fish that became the North Island, the sky father Ranginui sent two teardrops that became the Whanganui and Waikato rivers. According to tradition, the explorer Tamatea was the first to travel the river's full length, journeying upstream to Lake Taupo and leaving place names in his wake.

Before Europeans arrived, the river valley was densely populated, its banks lined with Maori kainga and pa. Over 200 rapids made navigation treacherous, but the Whanganui remained the primary route into the central North Island. In 1892, entrepreneur Alexander Hatrick launched the first regular steamboat service, contracted by Thomas Cook and Son to carry tourists upstream to Pipiriki. The journey became known as the Rhine of Maoriland, and thousands of visitors came each year to marvel at the forested gorges and riverside villages. A French filmmaker, Gaston Melies, shot a documentary along its banks in 1912-13, calling it the Rhine of New Zealand. But the completion of the North Island Main Trunk railway made the steamboat route obsolete. By the 1930s, failed farming schemes had left settlements abandoned, and one lasting monument to that era is the Bridge to Nowhere, a concrete span built to serve farms that no longer exist.

For Maori, the river is Te awa tupua, a living spiritual ancestor. Local iwi first petitioned Parliament for its protection in the 1870s, launching what would become the longest legal battle in the country's history. Waitangi Tribunal hearings in the 1990s, the Tieke Marae land occupation from 1993, and the Moutoa Gardens occupation of 1995 all kept the fight alive. On 30 August 2012, the government finally agreed to give the river its own legal identity. The resulting legislation, passed in 2017, appointed two guardians to speak for the river: one from the Crown, one from the iwi. The settlement did not merely resolve a property dispute. It enshrined in law a Maori worldview in which a river is not a resource to be owned but an ancestor to be respected.

The Whanganui remains one of New Zealand's richest ecological corridors. Native fish species include climbing galaxias, longfin and shortfin eels, pouched lamprey, and shortjaw kokopu. The Nankeen night heron established roosts along the river in the 1990s and breeds in New Zealand only here. Broadleaf and podocarp forest lines the banks, with crown ferns filling the understory. The paddle steamer PS Waimarie, one of Hatrick's original boats, has been restored and runs scheduled sailings from Whanganui. In 2010, the Adventurer 2 made the first voyage upstream to Taumarunui in 82 years. But the river's vulnerability was demonstrated in 1975, when a minor eruption from Mount Ruapehu spilled toxic crater lake water downstream, killing eels weighing up to 8.2 kilograms and trout up to 2.3 kilograms along its banks.