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 .