AbstractWildfires are common in Australia and can cause vegetation loss and affect hydrological processes such as interception, evapotranspiration, soil water storage and streamflow. This study investigates wildfire impacts on catchment mean annual streamflow for 14 Australian catchments that have been severely impacted by the 2009 Victoria wildfire, the second‐worst wildfire disaster in Australia. A statistical approach based on sensitivity coefficients was used for quantifying the climate variability impacts on streamflow and the time trend analysis method was used to estimate the annual streamflow changes due to wildfire respectively. Our results show that wildfire has caused a noticeable increase in mean annual streamflow in the catchments with a burnt area above 70% for an immediate post‐wildfire period (2009–2015) and the wildfire impact on streamflow is evidently larger than the climate change impact in the majority of burnt catchments. Furthermore, the wildfire impact on mean annual streamflow strongly increases with the burnt percentage area, indicated by R2 = 0.73 between the two. The results also illustrate that catchments with high burnt percentage areas can have more potential to gain increased streamflow due to wildfires compared with that due to climate variability and can have significant streamflow change after wildfires above the 70% threshold of burnt area. These results provide evidence for evaluating large‐scale wildfire impact on streamflow at small to medium‐sized catchments, and guidance for process‐based hydrological models for simulating wildfire impacts on hydrological processes for the immediate period after the wildfire.