The 25th Brigade was formed in the United Kingdom on July 1940, in response to the immanent threat of German invasion. Initially, it was only intended as a temporary formation and the troops for its three battalions were composed of 1,300 men from support units of the 6th Australian Division, and 459 infantry reinforcements intended for the infantry brigades in the Middle East. This manpower, however, was still only sufficient to form three rifle companies in each of the new battalions instead of the usual four. They were known as the 70th, 71st and 72nd Battalions but were later retitled the 2/31st, 2/32nd and 2/33rd Battalions respectively. In August it was decided that the 25th Brigade would become a permanent formation and in October it became part of the newly raised 9th Division. The brigade left the United Kingdom in January 1941, bound for the Middle East.
In a reorganisation of the AIF in February 1941, the 25th Brigade was transferred to 7th Australian Division and it fought under its command for the rest of the war. Its major campaigns were the invasion of Syria and Lebanon (June-July 1941); the advance on the Kokoda Trail and the destruction of the Japanese beachheads in Papua (September-December 1942); the capture of Lae (September 1943) and the Ramu Valley-Finsterre Mountains advance (September 1943-January 1944) in New Guinea; and the Balikpapan landing in Dutch Borneo (July-August 1945). The 25th Brigade disbanded in 1946.