The Grey River / Mawheranui 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. The river was named by explorer Thomas Brunner in honour of New Zealand politician Sir George Grey. The official name of the river was changed to Grey River / Mawheranui in 1998 by way of the Ngai Tahu Claims Settlement Act 1998. The Maori name for the river system and surrounding area is Mawhera, with Mawheranui being distinguished from the northern branch Little Grey River / Mawheraiti.