Thousands of Australians fled their homes as wildfires raged across the nation's south today, with firefighters struggling to contain the blazes fanned by strong winds.

Six homes were destroyed by the fires in South Australia and Victoria states, officials said, though no serious injuries have been reported.

Dry conditions and temperatures in the upper 30s Celsius were causing headaches for firefighters battling the blazes. Officials said it would take days to get the fires under control.

The worst of the fires was in the Adelaide Hills in South Australia, where the flames had destroyed five homes and put hundreds of others at risk, state Country Fire Service spokesman Daniel Hamilton said.

Residents of 19 communities had been asked to evacuate their homes as a predicted shift in the winds later today prompted fears the flames could worsen.

The forecast was also calling for increased humidity, which could slow the fire's progress, Hamilton said.

"There should be no sense of relief because the temperatures are somewhat lower, or that there are spots of rain about," South Australia premier Jay Weatherill told reporters in Adelaide. "The situation remains extremely dangerous and our warnings must be heeded."

Up to 2,000 firefighters were battling the blazes across the state from the ground and the air, with more than a dozen aircraft dumping water on to the flames. Six firefighters were treated for minor conditions, mainly smoke inhalation.

In neighbouring Victoria state, firefighters had controlled more than 300 fires since Friday, with only one still considered a danger. One home was destroyed in the blaze in the western Victoria town of Moyston.

Destructive wildfires are common across much of Australia during the summer months. In 2009, wildfires killed 173 people and destroyed more than 2,000 homes in Victoria.