It flows north from the Mamaku Plateau into Lake McLaren, to reach the Wairoa River 15 kilometres (9 mi) southwest of Tauranga.

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.

Te Rere I Oturu Falls are on the Opuiaki River, with the coordinates of 37° 55' 34.89" S 175° 59' 55.38" E. The name is based on a legend about a member of the Ngāti Ranginui tribe.

The river runs through, and is part of, the Opuiaki Ecological Area, created for the protection of native flora and fauna.

The Department of Conservation (DOC) carries out pest control and breeding programs, including for the kōkako, an endangered native bird species.