The Australian Imperial Force (AIF) was a expeditionary force created on 15 August 1914 for service overseas during the Great War. All members had volunteered to serve beyond the limits of the Commonwealth. The name was chosen by its first commander, Brigadier General W. T. Bridges, as representative of its dual Australian and Imperial mission. Today it is generally known as the First AIF, since a Second AIF was created for service in the Second World War.
Though it drew many of its personnel from the existing Permanent and Citizen Military Forces of the Australian Commonwealth Military Forces (AMF), the First AIF had a completely separate identity. All units of the First AIF were temporary units, raised for wartime service. After the war the battle honours of the First AIF were handed to AMF units.
Altogether some 328,583 Australian men and 2,131 women enlisted in the First AIF in Australia. Only 57 enlistments were accepted overseas. The Australian government attempted to minimise the number of AIF personnel serving with Imperial formations and vice versa.
Most AIF members went overseas as reinforcements earmarked to join certain units rather than as individuals. This changed in March 1918 when AIF personnel were sent as General Reinforcements, eligible to be drafted into any unit that needed them. Reinforcement drafts normally consisted of two officers and 150 other ranks. The Australian government usually offered to raise new AIF units or formations; the Imperial government in London would either accept or decline the offer. If the request to raise new units came from the theatre commander, the GOC AIF or the War Office, permission had to be sought from the Department of Defence in Melbourne.
More commonly new units were raised by Australian states. They were established at military bases located close to capital cities, from where the units embarked for overseas service. Formations comprised of units from multiple states were usually concentrated in Melbourne before embarkation. Other units were formed overseas from existing units and filled with reinforcements, and occasionally units were formed by 'Australianisation': i.e. Australian replaced personnel of British units.References:
Australian Defence Force Academy, University of NSW, website http://www.unsw.adfa.edu.au/~rmallett/