The Battle of the Hongorai River took place during the Second World War and involved Australian, New Zealand and Japanese forces. Part of the wider Bougainville Campaign of the Pacific theatre, the battle was fought in the southern sector of Bougainville Island. Coming after the Battle of Slater's Knoll in which a strong Japanese counterattack was defeated, the battle occurred in two distinct periods between 17 April and 22 May 1945, as elements of the Australian 15th Brigade advanced south along the Buin Road.