After the outbreak of war with Japan, the division was mobilised for war service and undertook defensive duties in southern Victoria, as part of Southern Command. In April 1942, the division became part of III Corps, which was responsible for the defence of Western Australia. The division occupied positions around Guildford, Geraldton, Moora and Perth. Following the arrival of the 2nd Division in July 1942, the division handed over the Perth region to them, although several rotations occurred over the next six months. In October 1942, the division took part in a large-scale anti-invasion exercise around Geraldton, which involved over 20,000 troops. Headquartered around Moora, the 2nd Brigade was positioned outside Geraldton, while the 6th held Irwin and Mingenew, and the 13th Brigade was around Dandaragan and Jurien Bay. Responding to a landing at Dongara, by a simulated Japanese division (played by the 8th Brigade) the exercise lasted four days and was the largest anti-invasion exercise undertaken by the Australian Army during the war.