Formed in 1912 as a Militia formation, it was later re-raised in 1916 as part of the all volunteer First Australian Imperial Force that was raised for overseas service during the First World War.

[3] Attached to the 4th Division and consisting of four infantry battalions—the 45th, 46th, 47th and 48th Battalions[4]—it served in the trenches of the Western Front from June 1916 onwards.

In 1921, the Australian government decided to reorganise the nation's part-time military forces in order to replicate the numerical designations and structure of the AIF.

[9] As a result, the 12th Brigade was re-raised at this time as part of the 6th Military District in Tasmania as a unit of the Citizens Force.

The brigade was mobilised for full time defensive service in December 1941, and was initially assigned to the defence of Hobart.

[16] As the war progressed, the size of the Darwin garrison was greatly reduced as other formations were disbanded or transferred to take part in combat operations in the Pacific.