The Opuiaki River is a river of the Bay of Plenty Region of New Zealand's North Island. It flows north from the Mamaku Plateau into Lake McLaren, to reach the Wairoa River 15 kilometres southwest of Tauranga. The river contributes to the Kaimai Hydro power scheme built in the 1980s. A dam was built up hill from where the Ngatuhoa Lodge is now located. From the dam a tunnel was dug,which channels the water into the Ngatuhoa Stream, where it enters just above another dam built there. Provisions are made to maintain some flow in both streams. Digging the tunnel was a challenging operation due to the geology of the area, formed by volcanic activity from various volcanoes, with clay and other sediment in between.