The name First Army was applied to at least two different Australian Army formations during World War II, both of them associated with Lt Gen. John Lavarack. In 1939, the name was used for a "skeleton", Militia formation based at Toowoomba, Queensland and comprising the 4th Division, 3rd Armoured Division, and the Torres Strait Force. As a Militia unit, the Army was reserved for home defence, and was therefore distinct from the Second Australian Imperial Force (AIF), raised to fight overseas. However, as was normal for the Australian Army at the time, unit names, formations and commanders above the battalion level were soon in a state of flux.

Following the beginning of the Pacific War, the threat of invasion by Japan, and the appointment of General Thomas Blamey as Commander-in-Chief of the Army, a major re-organisation took place, in April 1942. Lavarack was by now commanding I Corps, an AIF formation which was returning from the Middle East and North Africa. The First Army name was re-assigned to a formation built around I Corps, expanded above corps size with the addition of Militia units. The Army's initial area of responsibility was the defence of Queensland and northern New South Wales. (The Second Army was responsible for south-eastern Australia and the other components of Australia's defences were: III Corps (in Western Australia), the Northern Territory Force and New Guinea Force.)

In 1944-45, under Lt Gen. Vernon Sturdee, the First Army was based at Lae in the Australian territory of New Guinea. After a bar on the overseas deployment of Militia was relaxed, the First Army controlled many of the units which fought in the South West Pacific Theatre.

The First Army was disbanded in November 1945.