It flows for 170 kilometres (110 mi) from the Spenser Mountains (a northern range of the Southern Alps), firstly in a northwards direction and then northeast down a long, straight valley in inland Marlborough.

The river's lower reaches and surrounding fertile plain provide the basis for the Marlborough wine region.

The river has its outflow into Cook Strait at Cloudy Bay, just north of Blenheim in the island's northeast.

In pre-European and early colonial New Zealand, one of the South Island's largest Māori settlements was close to the mouth of the Wairau.

The Wairau Hydro Scheme proposed by TrustPower will operate on a 48-kilometre (30 mi) long canal.