Its catchment is bordered to the north by the Rangitata River and to the south by the Opuha, Waihi and Te Moana Rivers. As described by de Joux (1980), the river can be divided into two sub-catchments, based on topography: the mountainous upland catchment and coastal plains, respectively.

The upland catchment comprises an area 520 km² within the Canterbury Foothills, west of Geraldine. The Four Peaks, Ben McLeod and Tara Haoa Ranges make up the watershed in this region. The Phantom, Hewson and Mowbray Rivers that drain these hills constitute the main tributaries of the Orari River.

The Orari River emerges from the foothills, onto the coastal plain, at the Orari Gorge. From the Gorge, the river proceeds to flow south-eastwards 40 km (via the settlements of Orari and Clandeboye, and under SH79 and SH1 bridges), before discharging into the Pacific Ocean.