For twenty-four years Papua New Guinean units were a substantial part of the Australian Army, constituting the only regular force in Papua New Guinea (PNG) during the 1950s, and providing two battalions to secure the border with Indonesia during Confrontation. By 1972, shortly before Papua New Guinean independence, Papua New Guineans made up almost one in ten of the regular soldiers in the Australian Army. Just three years later, these men became the defence force of an independent PNG. Yet, there has been little scholarship that has addressed in detail the development of this force, the experiences of the soldiers within it, and its interaction with both the Australian colonial administration and PNG's independence in 1975. This thesis is the first study of the Australian Army in PNG to make extensive use of archival material and oral histories from both Australia and PNG. In doing so, it seeks to take advantage of under-utilised Papua New Guinean sources as well as the wealth of material that has only recently been released. This thesis explores the development of Papua New Guinean units from a colonial force to the independent Papua New Guinean Defence Force. It finds that while the Papua New Guinean units were initially raised overwhelmingly in the interests of Australian defence, the needs of an independent PNG were increasingly taken into account by the Army to such an extent that they coexisted as a secondary consideration for a significant period, becoming the Army's primary concern during the decade before independence. This thesis also examines the centrality of the treatment of Papua New Guineans to the development of the Army in PNG, showing that the changing perception of this unique group of soldiers shaped their role within the force. Finally, it questions the assumption that the Australian Army remained separate from wider developmental changes occurring in PNG during this period, revealing that the Army prepared for the possibility of Papua New Guinean nationhood even as it was tasked with defending Australia's interests. By examining the history of the Australian Army in PNG, and those who served within it, this thesis broadens our conception of how Australia was defended and of who defended it.