More than 2,700 people are without power in north central Victoria where temperatures are expected to exceed 40 degrees due to an underground cable fault.
Adelaide is forecast to hit 45C, and the Bureau of Meteorology says the city's 80-year-old record high of 46.1C is within reach. However, the fault may not be fixed until at least 5pm.
Fire crews are on stand-by while extra paramedics and ambulances will be on call to assist those struggling in the heat.