Plant–plant interactions are particularly complex in multi-resource limited environments. The aim of this study was to assess species interactions in estuarine wetlands where both N and fresh water are limited. We combined stable isotope methods and dissimilarity analyses to compare interspecific interactions in N source use and water source use. Both Melaleuca quinquenervia (Cav.) S. T Blake and Avicennia marina (Forssk.) Vierh. had a lower leaf δ15N when they were growing together with the N-fixer Casuarina glauca Sieb. ex Spreng. compared with those trees growing in monospecific stands, but their water isotopes, δ18O and δD, were different from C. glauca. Our results indicate that the N-fixer C. glauca shared their N with co-existing neighbours, either indirectly or directly, but that water sources were partitioned among them. Further analyses showed that M. quinquenervia and C. glauca had lower dissimilarity in N source use but higher dissimilarity in water source use than the C. glauca–A. marina pair, implying that the co-existence between M. quinquenervia and C. glauca is relatively stable. Our results suggest that facilitative interaction and resource partitioning can co-occur in estuarine wetlands, and which could be important in maintaining diversity across resource gradients.