The Ash Wednesday bushfires, known in South Australia as Ash Wednesday II, were a series of bushfires that occurred in south-eastern Australia on 16 February 1983, which was Ash Wednesday. Within twelve hours, more than 180 fires fanned by hot winds of up to 110 km/h (68 mph) caused widespread destruction across the states of Victoria and South Australia. Years of severe drought and extreme weather combined to create one of Australia's worst fire days in a century. The fires were the deadliest bushfire in Australian history until the Black Saturday bushfires in 2009.

In Victoria, 47 people died. There were 28 deaths in South Australia. This included 14 Country Fire Authority (CFA) and three Country Fire Service (CFS) volunteer firefighters who died across both states that day. Many fatalities were as a result of firestorm conditions caused by a sudden and violent wind change in the evening which rapidly changed the direction and size of the fire front. The speed and ferocity of the flames, aided by abundant fuels and a landscape immersed in smoke, made fire suppression and containment impossible. In many cases, residents fended for themselves as fires broke communications, cut off escape routes and severed electricity and water supplies. Up to 8,000 people were evacuated in Victoria at the height of the crisis and a state of disaster was declared for the first time in South Australia's history. More than 35 townhouses were burned in a small town in Victoria.

Ash Wednesday was one of Australia's worst fires. More than 3,700 buildings were destroyed or damaged and 2,545 individuals and families lost their homes. Livestock losses were very high, with more than 340,000 sheep, 18,000 cattle and numerous native animals either dead or later destroyed. A total of 4,540 insurance claims were paid totalling A$176 million with a total estimated cost of well over $400 million (1983 values) for both states or $1.3 billion in adjusted terms (2007).The emergency saw the largest number of volunteers called to duty from across Australia at the same time—an estimated 130,000 firefighters, defence force personnel, relief workers and support crews.