The Rai River is located in the Marlborough region of New Zealand's South Island, flowing into the Pelorus River. It is accessible from Havelock to the south or Nelson to the north, making it a relatively convenient destination. The river has been run at Grade 2, with flows ranging from low to high depending on conditions.

Rai Falls, located approximately 1.5 km from Pelorus Bridge, is a notable feature on the river and serves as one of the primary access points, with good parking available opposite the falls and an upstream walking track beginning near the parking area. Other vehicle access points include Bulford Bridge, where parking is available on the downstream side of the gravel road, Carluke Bridge with parking over the bridge on the downstream side, Brown Stream Reserve which offers both parking and camping, and a point on SH6 with safe parking near a house and a track to the river. The Tunakino Valley Bridge provides vehicle track access on the upstream side.

Water depth and speed can vary quickly on the Rai, so paddlers should be prepared for changing conditions. The river's flow is monitored at the Rai Falls gauge station, which provides real-time flow rate, water level, and rainfall data useful for trip planning.

The river passes through scenic country and features permanent pools, areas with sunken trees, and sections where the river cuts against banks, with willows and rock embankments present in places. The Rai also receives its tributary the Opouri River, which adds to the overall drainage of the system.