In volcanic islands, a crucial step for watershed resources management is the characterization of groundwater aquifers from local to regional scales. Airborne geophysical data provide high-resolution imagery down to hundred meters below the surface, over large territories. Yet, its regional accurate interpretation may be limited by the low density of field observations. In this study, we propose an approach combining airborne electromagnetic and magnetic data in order to resolve ambiguities and provide a multiscale hydrogeophysical characterization of Piton des Neiges volcano (Réunion Island). With limited calibration data, this methodology produces a geological model more robust and accurate than using airborne electromagnetic data alone. Through the continuous coverage of both methods, we demonstrate the influence of volcanic unit geometries on groundwater flows within the critical zone and we highlight major structures impacting groundwater flows at both local and regional scales.