1st Brigade is a combined arms formation of the Australian Army. Formed in 1903 as a militia formation based in New South Wales, it was reconstituted as part of the Australian Imperial Force in 1914 for service during World War I, the brigade fought at Gallipoli and on the Western Front before being disbanded in mid-1919. In 1921, the 1st Brigade was re-raised as a unit of Australia's part-time military forces, based in New South Wales. During World War II the brigade undertook defensive duties before being disbanded. In 1948, it was re-raised as an integral part of the Australian Regular Army. Currently the brigade is based at Robertson Barracks in Darwin and at RAAF Base Edinburgh near Adelaide, South Australia. It is the first of the Australian Army brigades to be re-organised as a combat brigade under Plan Beersheba.

Originally formed in 1903 as a Militia unit of the Commonwealth Military Forces, it was established in New South Wales and consisted of four battalion-sized units—1st, 2nd, 3rd and 4th Australian Infantry Regiments. In 1912, the compulsory training scheme was introduced and at this time, the brigade was reorganised as part of the 1st Military District, with constituent units were spread across various locations in Queensland including Townsville, Cairns, Charters Towers, Mackay, Rockhampton, Mount Morgan, Bundaberg, Maryborough, Gympie, and Brisbane.

The 1st Brigade was re-constituted in Sydney, shortly after the start of World War I in August 1914 for service overseas as part of the Australian Imperial Force (AIF). Upon formation it consisted of four infantry battalions—the 1st, 2nd, 3rd and 4th—however, later it received organic fire support when the 1st Australian Machine Gun Company (February 1916 to February 1918) and 1st Australian Trench Mortar Battery (from April 1916) were added to its order of battle. Assigned to the 1st Division, the brigade's first commanding officer was Colonel Henry MacLaurin.

During the war, the 1st Brigade took part in the fighting at Gallipoli between April and December 1915, before being evacuated to Egypt at the end of the campaign. There, it was brought back up to strength and in mid-1916 the brigade was transferred to Europe, where it took part in the fighting on the Western Front in France and Belgium between 1916 and 1918, before being disbanded in April 1919. Notable battles in which the brigade fought include: Lone Pine, Pozières, Bullecourt, Passchendaele, Hazebrouck, Amiens and the Hindenburg Line. Five soldiers from 1st Brigade units received the Victoria Cross, Australia's highest military decoration, for their actions during the war. These were: John Hamilton, George Howell, Thomas Kenny, Leonard Keysor and Alfred Shout.

In 1921, the Australian military part-time forces were re-organised to perpetuate the numerical designations and structures of the AIF. As a result, the 1st Brigade was re-raised as a part-time formation of the Citizens Forces based in Newcastle, New South Wales, and consisting of four infantry battalions: the 13th, 33rd, 35th and 41st Battalions.

Initially, the brigade was staffed through the compulsory training scheme, which meant that the brigade was able to maintain its numbers, however, in 1922, following the signing of the Washington Naval Treaty, Australia's security concerns were reduced. As a result, the Army's budget was halved and the scope of the compulsory training scheme was scaled back; with this the authorised strength of each infantry battalion was reduced to just 409 men of all ranks. In 1929, the compulsory training scheme was suspended by the newly elected Scullin Labor government and was replaced by a voluntary system, under the new name of the "Militia". This, coupled with the financial hardships of the Great Depression, meant that there were few volunteers available for service and many units had to be amalgamated or disbanded.

As a result, the 1st Brigade was reduced to just three infantry battalions. Upon the outbreak of World War II in September 1939, the 1st Brigade consisted of the 13th, 33rd and 41st Battalions. Initially, upon the commencement of hostilities it was decided to call up the Militia to undertake periods of continuous training to boost the nation's readiness for war, however, following Japan's entry into the war in December 1941, they were mobilised for defensive duties. Although some Militia units were committed to combat operations in New Guinea from 1942 onwards, the 1st Brigade remained in Australia for the duration of the war, headquartered around Parramatta, New South Wales, where it formed part of the 1st Division. Later the brigade became part of the Newcastle Covering Force and then the 10th Division. In 1942, the brigade moved to Sydney, and from 1943 it was reduced to meet operational manpower needs elsewhere. In September 1944, the brigade moved to Singleton, New South Wales, where the two of its three infantry battalions were disbanded. By the end of hostilities, it consisted of only one battalion—the 41st/2nd Battalion—as other units had been transferred, amalgamated or disbanded. Between May 1942 and August 1945 it was commanded by Brigadier Frederick Burrows.

Following the end of hostilities in August 1945, the existing structures were disbanded throughout 1945 and into 1946 and the Interim Army was raised. As a part of this, the 34th Brigade was raised for occupation duties in Japan. In 1948, with the establishment of the reformation of the Regular Army and the raising of the Australian Regiment (later known as the Royal Australian Regiment), the 34th Brigade was renamed the 1st Brigade following its return to Australia. During the Korean War, individual elements of the brigade were detached for combat in Korea, although they were subsequently placed under the command of other formations.