 Following the outbreak of World War II, due to the provisions of the Defence Act (1903) which precluded deploying the Militia outside of Australian territory, the government decided to raise an all volunteer force for overseas service, known as the Second Australian Imperial Force (2nd AIF). As such while the Militia would provide a cadre for this force, its main role was to provide training to conscripts as part of the compulsory training scheme which was re-established in early 1940. During this time the Militia were called up in cohorts for periods of continuous training and the 27th Battalion, along with the rest of the 3rd Brigade, undertook garrison duties in South Australia as well as running number of training camps designed to increase the nation's defences in case of war in the Pacific. With Japan's entry into the war following the attacks on Pearl Harbor and Malaya, the battalion was mobilised for wartime service on 9 December 1941 and transported to Darwin in northern Australia where it formed part of the garrison force to defend against possible Japanese attack. During this time it was based at Winnellie and took part in building defences in the north.
