The Waikà Âau River is a river in southern Fiordland, New Zealand. Since 21 June 2019 the official name has been Waikà Âau River. Waikà Âau River North Branch has been the official name of a tributary, which joins the river about a kilometre from the sea, since 19 December 2019. Rising north of the Hump Ridge, it flows south-eastward into Te Waewae Bay.
The New Zealand Ministry for Culture and Heritage gives a translation of "waters of the shag" for .
There is also a Waikoau River in Hawke's Bay, which flows into the Aropaoanui River.