As the existing militia forces were unable to serve overseas under the provisions of the Defence Act 1903 , an all @-@ volunteer expeditionary force known as the Australian Imperial Force ( AIF ) was formed and recruitment began on 10 August 1914 . The government pledged 20 @,@ 000 men , organised as one infantry division and one light horse brigade plus supporting units . Enlistment and organisation was primarily regionally based and was undertaken under mobilisation plans drawn up in 1912 . The first commander was Major General William Bridges , who also assumed command of the 1st Division . Throughout the course of the conflict Australian efforts were predominantly focused upon the ground war , although small air and naval forces were also committed .