On 9 July 1900, Royal Assent was given to the Commonwealth of Australia Constitution Act. On 17 September of that same year, an Order in Council approved a proclamation which declared that the union of New South Wales, Victoria, South Australia, Queensland, Tasmania and Western Australia should take effect from 1 January 1901.³³

On 1 March 1901, 28,923 colonial soldiers, comprised of 1,457 permanent, 18,603 militia and 8,863 unpaid volunteers, were transferred to the new Australian Army.³⁴ However, until the Defence Act of 22 October 1903, these troops continued to be administered under the relevant state acts. (Prior to federation, the separate Military Commandants of the Australian Colonies had already successfully lobbied their governments and the Colonial Office to allow their respective artillery formations to combine into a federal unit, which occurred on the 24^th of August 1899.)

Major General Sir Edward Hutton, who had previously commanded the New South Wales Military Forces from 1893 to 1896, took command of the Commonwealth Forces from 26 December 1901.³⁵ His recommendations for the structure of the new force were accepted by Government and, shortly before he relinquished his command in 1904, a Council of Defence, a Military Board of Administration, and an Inspector-General were established by the Defence Act of 1904.³⁶ The Act was further amended in 1908 to provide an additional member to the Military Board and to re-allocate duties among its members.

While compulsory military training had been debated since 1902, it was not until December 1909 that the necessary amendments were made to the Defence Act.³⁷ Before they could be implemented, however, Lord Kitchener was invited to report on Australian defences. His recommendations were embodied in further amendments to the Act in December 1910. As a result, in June 1911, the Royal Military College, Duntroon, was established.³⁸ With certain exceptions, Kitchener’s recommendations also resulted in a system of universal military training for junior and senior cadets from ages 12 to 18 and, thereafter, in the Citizen Military Forces (CMF) to age 26.³⁹ In 1911, 155,000 youths were registered, of whom 90,000 were in training, with 20,000 inducted each year until the outbreak of World War I.⁴⁰

Meanwhile, the development of aviation and its use in war had not passed unnoticed, and by 1912 the Military Board had approved the formation of the Australian Flying Corps and the establishment of a Central Flying School at Point Cook, Victoria.⁴¹
(The Royal Australian Air Force was later established as a separate service, in 1921.) ⁴²