This time they were deployed to Bougainville, where the Australian 3rd Division had taken over from the American garrison and the battalion joined the drive towards the Japanese stronghold at Buin in the south of the island.

[11] At the outset of the Second World War, due to the provisions of the Defence Act (1903) which prohibited sending the Militia to fight outside of Australian territory,[12][Note 4][Note 5] the decision was made to raise an all volunteer force, the Second Australian Imperial Force (2nd AIF), to serve overseas—initial operations were conceived to be likely in the Middle East, France and later possibly England—while it was decided that the Militia would be used to defend the Australian mainland from possible attack and to improve Australia's overall level of readiness through the reinstitution of compulsory military service and extended periods of continuous periods of training.

In December, after the Japanese attacks on Pearl Harbor and Malaya, they undertook defensive duties around Caloundra before moving to Townsville in May 1942 to defend against a possible invasion.

[6][18] In early 1942, the situation in the Pacific grew worse as the garrison at Singapore fell and the Japanese began to advance south towards Australia.

[6] With strong artillery support, the 61st and 25th Battalions managed to turn back the Japanese attack, inflicting heavy casualties and in the process seizing the initiative.

[24] They remained at Milne Bay until March 1943, however, when they were withdrawn to Donadabu, near Port Moresby where the battalion undertook training exercises and carried out patrols around the villages that were scattered around the start of the Kokoda Track.

[32] Initial Allied intelligence estimates placed the size of the Japanese forces on Bougainville at around 18,000 men, however, it was later determined that the number was closer to 40,000.

[34] Initially, the 7th Brigade, to which the battalion was attached, was employed in the northern sector holding positions along the Numa Numa trail, however, in January 1945 they took over from the 29th Brigade which had been operating in the southern sector and from then until March, the 61st Battalion took part in the advance inland from the Jaba River through Mosina, Sisirua, and Mosigetta towards the Puriata River.

[35] Throughout the rest of the month, the battalion was involved in a number of relatively minor engagements with the Japanese as they carried out patrolling and ambush operations along the Australian line of advance.

[37] In late February, the Japanese forces around Mosigetta withdrew after pressure from Captain Ted Hutchinson's company in an action for which he was later awarded the Military Cross.

[41] After this engagement the battalion was withdrawn from the line for a brief period due to concerns about its morale[6] while the rest of the 7th Brigade continued the advance along the Buin road throughout February and into March.

[42] Throughout March Japanese resistance steadily increased and during this time a number of attacks were made on the Australian rear areas and line of communications.

[43] As the Japanese launched a counteroffensive designed to check the Australian advance, the 61st Battalion's main engagement came between 29 March and 5 April when they took part in the Battle of Slater's Knoll.

[46] Over the course of the next week, the Japanese launched a number of attacks on the Australians around Slater's Knoll, during which time they suffered an estimated 620 men killed and over 1,000 wounded.

[47] The main Japanese attacks, however, were concentrated on the 25th Battalion and for the majority of the battle the 61st took part in defensive operations on the periphery of the 25th's position to protect their rear and to provide close infantry support to the Matilda tanks of the 2/4th Armoured Regiment.

[51] As problems with men refusing to go on patrol and lax performance of duty continued throughout early April, the 61st Battalion remained in the role of rear area security for the remainder of the 7th Brigade's involvement in the sector.

[56] As the battalion's strength dwindled as a result of the demobilisation process and men returned to civilian life, the decision was made to disband the unit.