The 1st Division, also known as the 1st (Australian) Division, is a division headquartered in Enoggera Barracks in Brisbane. The division was first formed in 1914 for service during the First World War as a part of the Australian Imperial Force (AIF). It was initially part of the Australian and New Zealand Army Corps (ANZAC) and served with that formation during the Gallipoli campaign, before later serving on the Western Front. After the war, the division became a part-time unit based in New South Wales. During the Second World War it undertook defensive duties in Australia. It was disbanded in 1945.

After the Second World War, the division remained off the Australian Army's order of battle until the 1960s, when it was reformed in New South Wales. In 1965 it adopted a certification role, determining the operational readiness of units deploying to Vietnam. It was re-formed in 1973 as a full division based in Queensland and in the decades that followed it formed the Australian Army's main formation, including both Regular and Reserve personnel. Throughout this period, the division's component units undertook multiple operations, mainly focused on peacekeeping in Africa, Asia and the Middle East.

Following the restructuring of the Australian Army under the "Adaptive Army" initiative, the 1st Division no longer had any combat units assigned to it, although the 2nd Battalion, Royal Australian Regiment became a direct command unit in late 2017. The division is tasked with co-ordinating the Army's high-level training activities and maintaining the "Deployable Joint Force Headquarters" (DJFHQ). In the event of the Australian Army undertaking a large-scale land-based operation, the division would have further combat units force assigned to it and would command all deployed assets including those of the Royal Australian Navy and the Royal Australian Air Force.

The Australian 1st Division was raised during the initial formation of the Australian Imperial Force (AIF) on 15 August 1914, shortly after the outbreak of the First World War. The division consisted of around 18,000 men, organised into three infantry brigades, each of four battalions, and various supporting units including artillery, light horse, engineers and medical personnel. Each infantry battalion initially consisted of eight companies, although in January 1915, they were reorganised into the British four-company system. Its first commander was the senior Australian general and head of the AIF, Major General William Bridges. Over the course of six weeks, the division's subordinate units were raised separately in the various states before embarking overseas. The transports then concentrated off the Western Australian coast and the combined fleet sailed for Britain. While en route, concerns about overcrowding in the training camps in the United Kingdom meant that the decision was made to land the division in Egypt, where it would complete its training before being transported to the Western Front.

While in Egypt, the division was assigned to the Australian and New Zealand Army Corps along with the New Zealand and Australian Division. Following the Allied decision to force a passage through the Dardanelles, the division was allocated to take part in a landing on the Gallipoli peninsula along with Anglo-French forces. The 1st Division made the initial landing at Anzac Cove on 25 April 1915. The 3rd Brigade formed the covering force which landed first, around dawn. The 1st and 2nd Brigades followed, landing from transports, and all were ashore by 9:00 am. While the landing was lightly opposed on the beach by elements of a single Turkish battalion, the Australians were checked short of their objectives as Turkish reinforcements arrived to secure the high ground around Chunuk Bair and Sari Bair. Critical fights developed on the left, over the hill known as Baby 700, and on the right on 400 Plateau, but stalemate set in and little further progress would be made for the remaining eight months of the campaign.

On 15 May 1915, after Bridges was mortally wounded by a sniper, an English officer, Brigadier General Harold Walker was given temporary command while a replacement was dispatched from Australia. This was Colonel James Legge, the Australian Chief of the General Staff, who was not an immediately popular choice with either his corps commander, Lieutenant General William Birdwood, or his subordinate brigade commanders. That same month, the division's artillery – three field artillery brigades each operating twelve 18-pound pieces, which had proved inadequate in the early battle, was boosted by the arrival of several Japanese-made trench mortars. They were later joined by several heavier guns including a 4.7-inch gun and two 6-inch howitzers. On 24 June, Legge replaced Walker, who returned to command of the 1st Brigade, but after Legge was evacuated from Gallipoli he was moved sideways to command of the newly formed Australian 2nd Division and Walker resumed command of the 1st Division.

The 1st Division's role in the August Offensive was to hold the front line and conduct a diversion on 400 Plateau at Lone Pine on 6 August. The resulting battle was the only occasion when a significant length of the Turkish trench line was captured, but resulted in heavy casualties. The main assault was made by the 1st Brigade, which was later reinforced by the 7th and 12th Battalions. Out of an assault force of 2,900 men, 1,700 were killed or wounded. On 7 August, the 6th Battalion from the 2nd Brigade made an unsuccessful attempt to capture the German Officers' Trench as a preliminary operation to other assaults by light horsemen at Quinn's Post and the Nek.

In October, Walker was severely wounded and replaced by the division's artillery commander, Brigadier General Talbot Hobbs who in turn fell ill and was replaced on 6 November by the commander of the Australian 1st Light Horse Brigade, Brigadier General Harry Chauvel. The 1st Division was evacuated from the peninsula in December, returning to Egypt. During the early months of 1916 the AIF underwent a period of re-organisation and expansion, and the division's experienced personnel were used to provide cadre staff to the newly formed 4th and 5th Divisions before being brought back up to strength in preparation for deployment to the Western Front. On 14 March, Walker, having recovered from his wounds, resumed command of the division, now part of I Anzac Corps. Seven members of the division received the Victoria Cross for their actions during the campaign: Alexander Burton, William Dunstan, Frederick Tubb, Patrick Hamilton, Leonard Keysor, Alfred Shout, William Symons.