Adelaide has set a new heat record, surpassing the previous one reached in 1939, hitting 46.6C this afternoon.
At least 20 locations hit record temperatures including Adelaide Airport, Minlaton, Noarlunga, Snowtown and Port Lincoln.
Ceduna set a record for the second day in a row, reaching 48.6C. At 3.15pm Port Augusta hit 49.1C.
Adelaide Metro said that due to the extreme heat it would run trams "at reduced frequency" between the Adelaide Entertainment Centre, west of the CBD, and the Adelaide Botanic Gardens.
Australia's Bureau of Meteorology (BOM) expected more records would fall later in the day.
Meteorologist Hilary Wilson said records were breaking across the state.
"It really is quite something to see this many records broken in one day," Ms Wilson said.
"It's preliminary at this stage but when we do the figures it wouldn't be at all surprising to see that this was the hottest day ever for South Australia as a whole.
"Typically the hottest time of the day is mid-afternoon so Adelaide could still sneak up a little bit higher."
The heatwave has also prompted fire bans in 13 districts.
SA Country Fire Services (CFS) state duty commander Yvette Dowling warned communities were at risk in the extreme heat.
"If a fire does start with these forecast weather conditions we're not going to be able to control that fire," she said.
"The communities are at risk, our firefighters are at risk, all emergency services are at risk."
Another Australian state, Victoria, was also expected to be hit by temperatures in the mid-40s.
About 2700 houses in the central Victorian towns of Nagambie and Avenel were set to battle an anticipated maximum of 41C during a power blackout.