The 18th Brigade was an infantry brigade of the Australian Army. The brigade briefly existed as a Militia formation prior to the First World War, but this was short-lived. During the Second World War, the brigade was raised on 13 October 1939 and was one of the first three infantry brigades of the Second Australian Imperial Force to be formed. Initially commanded by Brigadier Leslie Morshead, it served in the United Kingdom in 1940–1941, where it helped bolster the British garrison in anticipation of a possible German invasion following the Fall of France. In early 1941, the brigade was transferred to the Middle East where it later took part in fighting against the Italians in Libya and then helped to defend the besieged port of Tobruk before fighting against the Vichy French in the Syria–Lebanon campaign. The 18th Brigade was withdrawn to Australia in early 1942, and it later took part in the fighting against the Japanese in Pacific fighting several campaigns in New Guinea between late 1942 and early 1944. Its final involvement of the war came in mid-1945 when it took part in re-taking Balikpapan. Following the end of hostilities, the 18th Brigade was disbanded on 3 January 1946.

The 25th Brigade was a brigade-sized infantry unit of the Australian Army that served during the Second World War. Raised in July 1940 and consisting of three infantry battalions, the 25th Brigade initially served in the United Kingdom, where it formed part of the garrison tasked with defending against a possible German invasion. In 1941, the brigade was redeployed to the Middle East where it took part in the Syria–Lebanon campaign fighting several actions around Merdjayoun and Jezzine.