Coastal wetlands are dynamic ecosystems where freshwater and saltwater interactions are governed by both hydrological and oceanic processes. However, climate change and human interventions, such as lagoon breaching, increasingly disrupt these processes, posing challenges for ecosystem conservation. This study investigates the hydrogeological controls on water levels and salinity within a coastal wetland adjacent to Avoca Lagoon, an Intermittently Closed and Open Lake or Lagoon (ICOLL) in New South Wales, Australia. The wetland was specifically designed to provide breeding habitat for the endangered Green and Golden Bell Frog, which requires low-salinity conditions for breeding. To assess groundwater-surface water interactions and saltwater dynamics, we employed a multidisciplinary approach, integrating water level and salinity monitoring using surface water and groundwater piezometers at depths of 3.5 to 5.5 m, electrical resistivity tomography to characterise subsurface conditions, and bathymetric and salinity profiling of the lagoon. Results indicate strong hydraulic connectivity between the wetland and the lagoon, with groundwater discharge playing a critical role in sustaining freshwater conditions. While high lagoon levels can breach natural barriers and introduce brackish water into the wetland, low lagoon levels - resulting from controlled breaching - can cause wetland desiccation, reducing its suitability for frog breeding. These findings underscore the need for conservation strategies that balance flood management with habitat preservation. By advancing our understanding of ICOLL-influenced wetland hydrogeology, this research highlights the delicate interplay between hydrological processes, anthropogenic management, and frog habitat. Integrating groundwater dynamics into coastal wetland management will be crucial for sustaining ecological functions under future climate variability and sea level rise.