In January 1943, the 29th Brigade was deployed to New Guinea to serve as garrison troops in areas that had been recently secured by Allied forces. The 15th Battalion was initially sent to Milne Bay, remaining there for six months. It moved to Buna in July, but did not remain there long, transferring to Morobe a few weeks later. While there, the battalion's commanding officer, Lieutenant Colonel Jack Amies, appealed to the battalion to volunteer for service as part of the AIF. While some already had, the majority of the men were still classified as Militia personnel, which meant that the battalion was restricted in where it could serve and was liable to be broken up. To be classified as an AIF battalion, under the provisions of the newly passed "Defence (Citizen Military Forces) Act 1943", 65 per cent or more of the personnel had to volunteer for service outside Australian territory. The response was considerable and, by August, the battalion was gazetted as an AIF battalion. Later that month, as the Australians fought to secure Komiatum, the battalion moved on barges to Nassau Bay, and was committed to combat for the first time, occupying a position around Mount Tambu, known as "Davidson's Ridge", before marching to Tambu Bay. Arriving on 23 August, the battalion was initially placed into divisional reserve, as the 29th Brigade was committed to the drive on Salamaua.