the battalion was a militia unit at the outbreak of world war ii, attached to the 11th brigade. after the government decided to raise a separate force, the second australian imperial force (2nd aif), for service overseas due to the provisions of the "defence act (1903)" which precluded sending the militia overseas to fight, many members of the militia volunteered for service with the 2nd aif. as a result, the 51st battalion's numbers fell, however, in 1940 the national service scheme was reintroduced in effort to improve the nation's overall level of military preparedness and the battalion, its numbers swelled by an intake of conscripted soldiers, undertook a period of continuous training at miowera, near bowen.