Objectives: Limited human resources are widely recognised as an impediment to achieving the health-related Millennium Development Goals in Pacific Island Countries, with the availability of medical supplies and suitably trained health personnel crucial to ensuring a well-functioning medical supply chain. This paper presents our findings as we seek to answer the research question ‘What factors influence the availability of medical supplies within the health facilities of Papua New Guinea?’ Methods: We used a qualitative, triangulated strategy using semi-structured interviews, workplace observation and semi-structured focus groups. The parallel use of the interview tool and workplace observation tool allowed identification of ‘know-do’ gaps between what the interviewee said they did in their work practices, and the actual evidence of these practices. Focus groups provided further opportunities for raising and elaborating issues. Results: During 2 weeks of data collection we conducted 17 interviews and 15 observational workplace surveys in 15 facilities. Sixteen health personnel participated in 3 focus groups across 2 provinces and one district. An array of medical supply issues across all levels of the medical supply chain were revealed, including standard operating procedures, facilities, transport, emergency medical kits, the cold chain and record keeping. The influence of health worker training and competency was found to be common across all of these issues. Conclusion: The factors influencing the availability of medical supplies in PNG consist of a range of interrelating issues, consisting of both simple and complex problems involving the different levels and cadres of workers within the medical supply chain. Health systems sustainability theory suggests that a coordinated approach which addresses the inter-related nature of these issues, led by the PNG government and supported by suitable development partners, will be required for sustainable health systems change to occur. These changes are necessary for PNG to meet the health-related Millennium Development Goals.