Whanganui National Park was gazetted in 1986. The park protects one of the largest remaining tracts of lowland forest remaining in the North Island, and it provides habitat for a wide variety of native wildlife. Of all of New Zealand's national parks, Whanganui is most closely associated with human settlement. It is dissected by the Whanganui River, the longest continually navigable river in the country.
Although the bed and waters of the river are not included in the park, the river is an integral part of the area and provides an important access way into and through the area. Western tributaries of the Whanganui have eroded through mudstone (papa) which has created spectacular gorges, bluffs and a maze of intricate ridges and V-shaped valleys. Streams and rivers scattered throughout the park are an important habitat for the whio (blue duck), an endemic torrent duck admirably suited to river life.
Whanganui National Park contains extensive stands of podocarp-hardwood lowland forest, an ecosystem that is greatly under-represented in the New Zealand conservation estate. This extensive forest comprises many different communities reflecting changes from north to south, riverside to high ridges.
Dry ridges are a distinct habitat with black beech over twiggy shrubs. Autumn orchids are also commonly seen on these ridges. Silver and hard beech can also be found in the park. The steep riverbanks are another distinctive feature and are a haven for some of the park's more vunerable plants, like hutu, fuchsia and the rare daisy brachiglottis turneri. Introduced plants are mostly limited to riverbanks, walking tracks and heavily disturbed areas, but weeds that are a problem in some areas include buddleia, wattles, willows, Japanese honeysuckle, pines, Japanese walnut and to a lesser extent gorse and Himalayan honeysuckle.
The isolated reaches of the Whanganui River valley support a range of birds. The greatest concentration of birdlife in the park is in the area most intensively managed to reduce the effects of exotic pests. Common species that can be seen include fantails, grey warblers, silvereyes, tomtits, robins, bellbirds, kererū/kūkupa, tūī and whitehead. Migratory cuckoos are heard in spring and early summer. Kākāriki and kākā are present in very low numbers in the mature forest of the Matemateaonga Range and possibly the Heao catchment. Rifleman, yellow-crowned käkäriki and New Zealand falcon are seen regularly.