The Grey River / Māwheranui is located in the northwest of the South Island of New Zealand. It rises in Lake Christabel, one of numerous small lakes on the western side of the Southern Alps,12 kilometres southwest of the Lewis Pass, and runs westward for 120 kilometres before draining into the Tasman Sea at Greymouth. Thomas Brunner, who explored the area in the late 1840s, named the river in honour of Sir George Grey, who first served as Governor of New Zealand from 1845 to 1854. The Ngai Tahu Claims Settlement Act 1998 changed the official name of the river to Grey River / Māwheranui in 1998. The Maori name for the river system and surrounding area is Māwhera, with Māwheranui being distinguished from the northern branch Little Grey River / Māwheraiti.