The Black Friday fires of January 13, 1939, in Victoria, Australia, are considered one of the worst natural bushfires (wildfires) in the world, and most certainly the single worst in Australian history as a measure of land affected. Almost 20,000 km² (4,942,000 acres, 2,000,000 ha) of land was burnt, 71 people died, several towns were entirely destroyed, and the Royal Commission that resulted from it led to major changes in forest management. Over 1,300 homes and 69 sawmills were burnt and a total of 3,700 buildings were destroyed. It was calculated that three quarters of the State of Victoria was directly or indirectly affected by the disaster. The Royal Commission into the fires was to note, "it appeared the whole State was alight on Friday, 13 January, 1939".
The summer of 1938–39 had been hot and dry, and several fires had broken out. By early January, fires were burning in a number of locations across the state. Then, on Friday January 13, a strong northerly wind hit the state, causing several of the fires to combine into one massive front.
Ash from the fires fell as far away as New Zealand. The fires came under control two days later, when rain fell on the night of Sunday the 15th.
The subsequent Royal Commission, under Judge L.E.B Stretton, attributed blame for the fires to careless burning, such as for campfires and land clearing. It made a number of recommendations to improve forest management and safety, such as the construction of fire towers and access trails. It also encouraged the creation of a regime of supervised burning, which still exists today.
The fires contributed directly to the passing of the Forests Act, which gave the Forests Commission responsibility for forest fire protection on public land. They were also a key factor in the founding of the Country Fire Authority in 1944.
The environmental effects from the fires continued for many years, and some of the burnt, dead trees still remain today. Large amounts of animal habitat were destroyed. In affected areas, the soil took decades to recover from the damage of the fires. In some areas, water supplies were contaminated for some years afterwards, due to ash and debris washing into catchment areas.