The introduced Cane Toad Rhinella marina has recently expanded its range into the monsoonal north of Australia near Darwin, Northern Territory. Aggregated toads were collected toward the end of the prolonged dry season from an isolated and localized occurrence of moist soil, where they were observed in typical water absorbing postures. Water potential of the moist soil (−31 kPa) was sufficient that toads could extract liquid water via the ventral pelvic patch, but surrounding dry soils with water potentials lower than −8,000 kPa could not be used as moisture sources. High mean plasma (358.5 mOsm kg⁻¹) and urine (353 mOsm kg⁻¹) osmolality values are indicative of dehydration, and high urea concentrations in plasma (109 mmol L⁻¹) and in urine (237 mmol L⁻¹) demonstrate accumulation and retention of waste nitrogen. All parameters differed markedly from those of fully hydrated toads in the laboratory and from active animals collected in the wet season. The urine osmolality of wet season individuals (mean ± SD: 118.9 ± 76.4 mOsm kg⁻¹) was intermediate compared to the laboratory hydrated animals and the dry season sample, but plasma osmolality and urea concentrations were similar to fully hydrated toads. Differences in body fluid osmolality reflect the availability of soil and surface moisture in the environment in the wet and dry seasons. The need to access residual moisture sources during the late dry season is likely to limit Cane Toad movement and resulted in the aggregation of toads at the rehydration site. This enforced aggregation potentially facilitates effective control of toads at a local scale.