The Hunter River rises on the western slopes of Mount Royal Range, part of the Liverpool Range, within Barrington Tops National Park, east of Murrurundi, and flows generally northwest and then southwest before being imponded by Lake Glenbawn; then flowing southwest and then east southeast before reaching its mouth of the Tasman Sea, in Newcastle between Nobbys Head and Stockton. The river is joined by ten tributaries upstream of Lake Glenbawn; and a further thirty-one tributaries downstream of the reservoir. The main tributaries are the Pages, Goulburn, Williams and the Paterson rivers and the Moonan, Stewarts and Wollombi brooks. East of Hexham, the river splits into two main channels, separated by the Ramsar-protected Kooragang Wetlands that feeds Milham Ponds, Wader Pond, Swan Pond and a series of smaller wetland pondages. The southern arm of the river also creates Hexham Island, while the northern creaters Smiths Island and flows in Fullerton Cove. The two channels converge at Walsh Point, reaching confluence with Throsby Creek adjacent to the Newcastle central business district, before reaching the river mouth. The Hunter River descends 1,397 m (4,583 ft) over its 468 km (291 mi) course from the high upper reaches, through the Hunter Valley, and out to sea.
The Hunter River is subject to substantial flooding, which Glenbawn Dam, near Scone, was constructed to ameliorate. Major floods have occurred on the Hunter including the flood of 1955 that caused devastation to townships along the river, especially Maitland. Severe flooding again occurred in June 2007 and again in 2015.
The Hunter Valley is one of the best routes to the interior of the state with access relatively unimpeded by mountains and other obstacles. It is the largest area of relatively low-lying land near the coast of New South Wales, and owing to the shielding by rugged ranges to its north, is much drier than any other coastal region of the state. Annual rainfall ranges from 1,100 mm (43 in) at Newcastle to only 640 mm (25 in) at Merriwa and Scone in the upper reaches. In the driest years rainfall can be as low as 600 mm (24 in) at Newcastle and 375 mm (15 in) in the upper valley.
Around the Barrington Tops on the northern side of the valley, however, annual precipitation can be as high as 2,000 mm (79 in), not all of which falls as rain since July temperatures are often below 0 °C (32 °F). In the lower areas, summer maxima are usually around 27 °C (81 °F) and winter maxima around 16 °C (61 °F).
Except for the driest parts of Tasmania and a small area of the Monaro between Cooma and Nimmitabel, the Hunter Valley is the southern limit of rich "black earths" (actually black cracking clays). These are the only soils in all of Australia with reasonable levels of soluble phosphorus, with the result that upstream from Singleton very rich pasture land with many thoroughbred horse studs occurs. Around Merriwa and south of Singleton, the soils are very infertile sands more typical of Australia as a whole, and the dominant land use is extensive grazing.
The Hunter River was discovered by European explorers in the 1790s. In June 1796 fishermen sheltering from bad weather discovered coal there, and the river was initially called Coal River. In 1797 it was formally named the Hunter, after Captain John Hunter who was Governor of the British colony in New South Wales at that time.