Following the outbreak of World War II, the 20th/19th Battalion was transferred from Sydney to Darwin in the Northern Territory, where they were to form part of the town's garrison. These two battalions were delinked on 1 November 1941 as part of the expansion of the Australian military following Japan's entry into the war. The 20th/19th Battalion's machine gunners later formed part of the 6th Machine Gun Battalion. At the split, the 20th Battalion was assigned once again to the 5th Brigade, although in February 1942 they were transferred to the 28th Brigade. With the 28th Brigade, the 20th Battalion undertook defensive duties around Port Kembla and Seaham in New South Wales. Nevertheless, as a result of a manpower shortage in the Australian economy, the government decided to disband or amalgamate a number of Militia units throughout 1942–43, and on 13 December 1943, the 20th merged with the 34th to form the 20th/34th Battalion. This unit remained in existence until it was disbanded on 6 April 1944. Although the 20th Battalion received no battle honours for direct involvement in combat during World War II, in 1961 it was entrusted with the three battle honours bestowed upon the 2/20th Battalion for their involvement in the Malayan Campaign.