Aim: Megafire events generate immediate concern for wildlife and human well-being, but their broader ecological impacts likely extend beyond individual species and single fire events. In the first mechanistic study of fire effects focussed on ecosystems, we aimed to assess the sensitivity and exposure of ecosystems to multiple fire-related threats, placing impacts in the context of changing fire regimes and their interactions with other threats. Location: Southern and eastern Australia. Time period: 2019–2020. Major species studied: Australian ecosystems. Methods: We defined 15 fire-related threats to ecosystems based on mechanisms associated with: (a) direct effects of fire regime components; (b) interactions between fire and physical environmental processes; (c) effects of fire on biological interactions; and (d) interactions between fire and human activity. We estimated the sensitivity and exposure of a sample of 92 ecosystem types to each threat type based on published relationsh