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.