The Commonwealth of Australia came into existence on 1 January 1901 as a result of the federation of the Australian colonies . Under the Constitution of Australia , defence responsibility was now vested in the new federal government . The co @-@ ordination of Australia @-@ wide defensive efforts in the face of Imperial German interest in the Pacific Ocean was one of driving forces behind federalism , and the Department of Defence immediately came into being as a result , while the Commonwealth Military Forces ( early forerunner of the Australian Army ) and Commonwealth Naval Force were also soon established .