Heatwaves negatively impact wildlife populations and their effects are predicted to worsen with ongoing global warming. Animal mass mortality at extremely high ambient temperature (Ta) is evidence for physiological dysfunction and, to aid conservation efforts, improving our understanding of animal responses to environmental heat is crucial. To address this, I measured the water loss, body temperature and metabolism of an Australian marsupial during a simulated heatwave. The body temperature of the common ringtail possum Pseudocheirus peregrinus increased passively by ∼3°C over a Ta of 29–39°C, conveying water savings of 9.6 ml h−1. When Ta crossed a threshold of 35–36°C, possums began actively cooling by increasing evaporative water loss and thermal conductance. It is clear that facultative hyperthermia is effective up to a point, but once this point is surpassed – the frequency and duration of which are increasing with climate change – body water would rapidly deplete, placing possums in danger of injury or death from dehydration.