Dissolved organic matter (DOM) in a tropical estuarine ecosystem was characterized through the analysis of optical properties of coloured and fluorescent matter in porewater samples collected from the coastal areas of New Caledonia (South-West Pacific Ocean). Surface sediment porewaters were analysed in five sites under the influence of catchment areas characterized by different proportions of ultramafic soils. Four main components were identified, comprising both autochthonous and allochthonous materials across all sites. Significant site-specific effects were driven by three main factors: geology, catchment area size, and mangrove presence. Ultramafic sediments impacted more or less by mining activities resulted in low coloured dissolved organic matter content and high biological activity. Larger catchment areas led to high concentrations of terrestrial humic-like and fulvic-like compounds. The presence of mangroves influenced the abundance of humic-like-mangrove compounds. More broadly, our research highlights the importance of considering the entire land-sea continuum and the complex interactions between terrestrial and marine ecosystems to fully understand the dynamics of estuarine environments.