Environmental policymakers need to monitor the state of the environment to evaluate the effectiveness of their actions, prioritize policies, and thus establish their contribution to the conservation of natural capital. The Environmental Sustainability GAP (ESGAP) is a framework that introduces a synthetic dashboard to monitor the state of the environment from a strong sustainability perspective, adopting stringent criteria on the maintenance of critical natural capital. This article focuses on the implementation of the ESGAP framework in New Caledonia, a developing economy heavily dependent on natural capital. The results highlight sustainability issues that require policy attention and methodological challenges about the construction of sustainability indicators. The strong environmental sustainability index score obtained is 43/100. This value is mainly driven downwards by high greenhouse gas emissions, and the impact of fire on ecosystems in New Caledonia. Both are moving away from the strong environmental sustainability objectives over the years, indicating a worsening situation. The ESGAP provides a holistic view on sustainability that was previously lacking, thus informing environmental management and policies. This study also identifies gaps in environmental monitoring and needs in the setting of objectives, as well as the cost of reaching them.