Recent forest fires potentially intensify flood hazards. However, forest fire amplification of floods is not well understood at a large scale due to the complex compound impacts of forest fires and climate variability, while available small-scale cases may not represent regional changes. Here, we show that the 2019–2020 mega forest fires in southeast Australia, with unprecedented burned areas, significantly (p < 0.05) increased the peak discharges of floods during the 2 years after the fires. Spatially, fire impacts on these floods are much stronger in regions with winter-dominated and uniform rainfall but insignificant in regions with summer-dominated rainfall. The regional divergence reveals that burned areas can aggravate floods by exacerbating infiltration-excess runoff processes but may not exert significant effects where saturation-excess processes dominate. People may be increasingly exposed to such flood hazards, especially in regions where forest fires have become more frequent under climate change.
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