Varanus mertensi is a semiaquatic lizard that lives near permanent water in northern Australia. During the wet and dry seasons, we measured the field metabolic rate (FMR) and water flux rates of animals in the field and standard metabolic rates across a range of body temperatures (18-36 C) in the laboratory. We combined these data to divide the FMR into energy expended during periods of rest and activity. The FMR was significantly higher in the wet season (120.7 kJ kg-1 day-1) than in the dry season (81.1 kJ kg-1 day-1). There was no difference in the water flux rate between the wet (63.2 ml kg-1 day-1) and dry (66.5 ml kg-1 day-1) seasons. The FMR during the wet season is greater than that predicted for a similarly sized iguanid lizard, but the 95% confidence interval around the mean FMR in the dry season overlaps the predicted iguanid value. The calculated percent of the FMR devoted to activity is high for both seasons (70-73%) compared to other lizards, as is the calculated sustainable metabolic scope (3.4-3.7). When compared to terrestrial varanid lizards living in the same area during their active seasons, V. mertensi does not show any quantitative differences in FMR or water flux. Varanus mertensi does differ from these terrestrial species in not having an inactive period during the dry season, and we suggest that this is related to the continual availability of food and water in its habitat.