More than 100 schools across South Australia (SA) have been closed as the Australian state braces for catastrophic fire danger day.

Temperatures in some parts of the state were forecast to soar above 40 degrees Celsius on Wednesday, accompanied by strong winds, the UNB reported.

About seven districts were declared catastrophic fire danger zones by the state government, and a state-wide total fire ban was put in place.

"From sunrise until well past midnight, this state is going to experience very difficult fire conditions," said Brenton Eden, Country Fire Service (CFS) assistant chief officer, according to the Australian Broadcasting Corporation (ABC).

With 10,000 firefighters ready in the CFS, he said those conditions posed challenges for the fire service. "As people will have seen in New South Wales (NSW), should a fire start, the fire activity will be intense. It will be fast and it will be difficult to control."

Four people were killed by bushfires in NSW recently which also destroyed more than 500 homes and burnt more land than any other fires in the state in the last 25 years.

Eden said the conditions on Wednesday were expected to be the worst in SA since 2015.

"These are the worst fire conditions we've experienced since going back to Pinery four years ago where we lost 85,000 hectares and (had) two fatalities in that tragic accident," he said.