The Opuiaki River is a 24-kilometre river in the Bay of Plenty Region of New Zealand's North Island, flowing north from the Mamaku Plateau into Lake McLaren before reaching the Wairoa River approximately 15 kilometres southwest of Tauranga. Its flow has been significantly reduced since the 1970s when most of its water was diverted into Mangapapa Lake as part of a hydroelectric development, which fundamentally altered its character as a waterway.

Kayaking on the Opuiaki is strictly for advanced paddlers and is dependent on rain events, as normal flows are insufficient to provide paddling amenity. The whitewater section begins at McLaren Falls, a seven-metre Grade VI waterfall that is generally not paddled. Below this, the first one-metre drop known as "Humpty Dumpty" is often used as a warm-up. The first major rapid, "Mother's Nightmare," is a long Grade IV rapid finishing with a two-metre drop. This is followed by several Grade III rapids — Helicopter, Double Trouble, and Devils Elbow/Washing Machine. The two Grade V sections are "Top Waterfall," a small drop into a hole followed by a three-metre drop, and "Roller Coaster." Downstream, paddlers encounter a Grade III rock garden, a Whitewater Slalom gorge, further Grade III rapids, and the "Bottom Waterfall," another two-metre drop.

From Lake McLaren, the lower portion of the Opuiaki can be paddled by entering from the lake and heading upstream into the river mouth. There is no public access to the upper catchment, and access to the river generally requires crossing private land, so landowner permission must be sought. The Opuiaki is accessed off Soldiers Road from State Highway 29, just beyond the Ngamuwahine bridge.

Given the reliance on rainfall to bring the river up to paddleable levels and the advanced nature of the rapids, this is not a suitable run for intermediate paddlers or those unfamiliar with high-consequence whitewater. The steep, fast nature of the upper river under flood conditions makes it extremely hazardous and not recommended for any paddling attempt.