The Mokau River, which is a catchment area of 550 sq. miles, rises on the south-western slopes of the Rangitoto Range. Its upper catchment adjoins that of the Wanganui in the south, and the Waipa, a major tributary of the Waikato, in the north. The lower course of the Mokau River lies through rugged country rising to almost 2,000 ft, much of it clad in dense native rain forest. This lower course lies through easily eroded tertiary sandstones and mudstones, within which occurs coal of the Mokau coalfield. For several miles south of Piopio, a small town some 12 miles south of Te Kuiti, the Mokau River flows through limestone-capped hills with greywackes of Jurassic and Triassic age outcropping in its bed. The river falls rapidly from the area of more-resistant rocks to the softer ones, where during the last glaciation, when the sea was some 350 ft below present level, the valley floor lay considerably lower than it does now. Sea level rose with the melting of the continental glaciers and the Mokau River aggraded its valley floor. The lower valley is still subject to extensive flooding.
At its mouth the Mokau River is confined by a sand bar, and on the raised beaches formed during past interglacial periods, ironsand dunes have been deposited. The river carries a predominantly fine sediment load, has a low gradient over the last several miles of its course, and is tidal for some distance upstream. From near Piopio the river enters into a broadly open, low-rounded topography above the steep-walled, narrow limestone gorges.
In pre-European times the Mokau River marked the boundary between the Tainui and Taranaki tribal areas; in particular, the territory was often under dispute between Ngati Maniapoto and Ngati Tama. Although a small tribe, Ngati Tama, held their own against all comers until the early part of the nineteenth century when two battles altered the traditional power balance in the district. About 1815 there was a disastrous battle, Nga-tai-pari-rua, against Ngati Rakei on Mokau beach. Six years later a strong Maniapoto war party, armed with muskets, invaded the district. There was a fierce engagement at Pararewa in September 1821, when the Ngati Tama and Te Ati-awa were heavily defeated and Tupaki, the great Ngati Tama war chief, was killed. After this the remnants of Ngati Tama joined Te Rauparaha's trek south to Cook Strait. Their departure left Taranaki open to the depredations of Te Wherowhero and his Waikatos.
The river for its whole length once formed the boundary between the Auckland and Taranaki Provinces, and today forms the boundary between the land districts The port, Mokau, at the mouth of the river, is no longer used.
The meaning of the name is obscure. One variant is “winding stream”; another, that the river was named Mokau by Turi from his having slept there.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.