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. Plain Language Summary: Wildfires and floods are both natural hazards that cause damage to lives and properties. Understanding how these two are linked is critical not only for catchment hydrological processes but also for hazard control and water resources management. However, it is unclear whether mega forest fires can influence the subsequent floods on a large scale. This study demonstrates that the recent mega forest fires in southeast Australia have significantly amplified floods for most fire‐impacted catchments. Apart from raising awareness of global fire activities, this study indicates that people living in the fire‐prone area may be also susceptible to more flood hazards after fires in the future. Key Points: The unprecedented 2019–2020 forest fires intensified subsequent river flood magnitudes in southeast AustraliaForest fire amplification of floods is divergent in different regionsRegional divergence in fire impacts is attributed to different burned areas and dominant flood generating mechanisms [ABSTRACT FROM AUTHOR]