The 31st/51st Battalion was an infantry battalion of the Australian Army, which served during World War II. Raised for service as part of the Militia in 1943 through the amalgamation of two previously existing battalions, the 31st/51st Battalion undertook garrison duties in Dutch New Guinea in 1943–44 before taking part in the Bougainville Campaign in 1944–45. Following the end of the war, the battalion served in the Pacific overseeing the transfer of Japanese prisoners of war and re-establishing law and order until mid-1946 when it returned to Australia and was disbanded.