Crews fighting serious bushfires in Australia have downgraded the situation from 'emergency' to 'watch and act'.

Thousands of people in South Australia and Victoria have had to flee their homes in the past 24 hours.

South Australia's fire chief said the blazes in the Adelaide Hills, northeast of Adelaide city, were the worst since the Ash Wednesday bushfires in 1983.

Cooler conditions are helping the situation, but Channel 9 reporter Samantha Vadas says the fire has already burned through 11-thousand hectares of land.