Ōpanuku Stream runs approximately 11 kilometres from its source on the Scenic Drive ridge in the Waitākere Ranges eastward through Henderson Valley, then north-east through the West Auckland suburbs of Western Heights and Henderson, before joining the Oratia Stream to form Te Wai-o-Pareira/Henderson Creek, which flows into the Waitematā Harbour. The stream drops off rocky terrain in its upper reaches before levelling out through the valley floor.

The character of the stream varies considerably along its length. Upper and middle sections through Henderson Valley can be too shallow for uninterrupted paddling, and accounts of trips along the full course describe the need for ropes as well as paddles, indicating portaging or lining through shallower or more technical passages. The lower reaches approaching the confluence with the Oratia Stream are more consistently navigable, and the short stretch from that confluence onward to Henderson Creek has been described as straightforward paddling.

A notable landmark along the route is the site of Henderson's Mill, built in 1849 and closed in 1866 after the accessible kauri timber was exhausted. The mill cookhouse survives as a heritage cottage, and a replica water wheel marks the location. The confluence of the Ōpanuku and Oratia Streams was historically a landing point, and Henderson Creek served as the primary transport link to Auckland until the 1890s.

The stream and its surroundings have undergone active ecological restoration since the mid-2000s, with replanting of native flora. Wildlife present in and around the stream includes tuna (eels), koura (crayfish), and various native birds. Water quality has been affected historically by pollution, deforestation, and urban development, though restoration work is ongoing. A walk and cycleway runs alongside much of the stream corridor through the Project Twin Streams area, spanning from Candia Road in the upper reaches to Corban Estate in the lower reaches.