It rises 12 kilometres southwest of the Lewis Pass in Lake Christabel, one of numerous small lakes on the western side of the Southern Alps, 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.

[2] The New Zealand Ministry for Culture and Heritage gives a translation of "broad and widespread (river mouth)" for Māwheranui.

The Grey River's mouth is protected by a large sandbar, Greymouth bar, which is a notorious danger to shipping.

Historically, sewage and stormwater from Greymouth, Cobden and Blaketown were discharged without treatment directly into the Grey River.