Firefighters in South Australia declared a catastrophic fire danger for two of the state's northern districts Wednesday.

It was the first time the catastrophic rating has been issued in Australia under the national fire warning system established in the wake of February's Victoria bushfires, which killed 173 people.

The new warning system was brought in to try to prevent a repeat of the horrendous loss of life in the Black Saturday bushfires that swept through Victoria. A catastrophic fire risk indicates the highest danger.

The weather bureau issued the rating for the Flinders and North-West Pastoral districts in the north and west of the state late Tuesday because of the predicted extreme temperatures, strong winds and low humidity. Some parts of Australia have been reaching 40 C or more.

Officials early Thursday downgraded the warnings to severe, third-highest on the system.

The Country Fire Service said if fires had broken out Wednesday, they would have been uncontrollable and people would have died. Eleven schools and preschools that are rated as unable to withstand a serious fire closed Wednesday as a result of the catastrophic rating.

The state's aerial fire-fighting fleet will be available for use two weeks earlier than usual because of the hot and dry conditions.

The state government said water-bombing surveillance planes could be activated immediately if needed.