Compulsory military service was effectively introduced in 1942, when all available men 18-35, and single men aged 35-45, were required to join the Citizens Military Forces (CMF) to defend Australia. In early 1943 'Australia' was defined officially to include New Guinea 'and such other territories in the South-west Pacific Area as the Governor-General proclaims as being territories associated with the defence of Australia'.
CMF militia units were involved in fierce fighting against the Japanese advance on the Kokoda Track in New Guinea (then an Australian territory). Compulsory military service ended in 1945, and most Australian personnel had been demobilised by the end of November 1946.
Wikipedia, 'Conscription in Australia', http://en.wikipedia.org/wiki/Conscription_in_Australia
Australian War Memorial encyclopaedia, 'Conscription'