By living in close proximity to waterbodies, semi-aquatic varanids may be able to sustain activity and maintain relatively smaller home ranges than their terrestrial counterparts because of access to important resources (food and water) that are less abundant in terrestrial systems during dry seasons. The factors influencing home range and movement were examined in two semi-aquatic Australian varanids, Varanus mertensi and Varanus indicus. We used radio-telemetry and mark–recapture techniques to examine differences in space use between sexes, seasons and body sizes in the top end of the Northern Territory of Australia. Both V. mertensi and V. indicus were active throughout the year, unlike terrestrial varanids in this region. Varanus indicus had small home ranges when compared with other similar sized varanids, whereas the home ranges of V. mertensi varied substantially. Body size influenced the home range of V. indicus, with larger animals having larger home ranges, but not for V. mertensi. An interaction between body size and gender influenced the distance V. indicus moved, whereas interactions between body size, gender and seasonality all influenced the distance that V. mertensi moved.