SYDNEY (REUTERS) - Hundreds of firefighters are battling to contain Australia's worst wildfires in 30 years as the danger is expected to rise with soaring temperatures and changing winds on Jan 7.

The fires burning across a 240km perimeter in South Australia state have already swept across more than 12,000ha and destroyed at least 26 homes and 41 outbuildings. At least 29 people have been injured or hospitalised but no fatalities have yet been recorded.

The fires are the worst in terms of scale and intensity since the Ash Wednesday fires of 1983, which killed 75 people and destroyed more than 2,000 homes.

Country Fire Service state coordinator Brenton Eden said the coming 48 hours were critical to fire-fighting efforts because of the higher forecast temperatures and changing winds.

"Today we will see easterly winds coming through and by mid-afternoon, 4pm, we're going to be back to 36 deg C," Mr Eden said on Tuesday.

"We've got to get through Tuesday and Wednesday and see no increase in the perimeter of this fire."

Hundreds of families have been evacuated and firefighters have rescued scores of koalas in the fire that is raging through the Adelaide Hills, just outside the state capital.

Wildfires are a natural annual event in Australia, but scientists say climate change is increasing both the fire season and intensity.

The CFS warned that the fires could flare up for the next few weeks.

Meanwhile, the hot weather sweeping the country caused different problems in the West Australian capital of Perth, where thousands of iiNet Internet customers found themselves offline for hours after the company shut down some of its systems.

The temperature in Perth reached a maximum of 44.4 deg C on Monday, the hottest January day since 1991, when it reached 45.8 deg C on Jan 31.

"Due to record-breaking temperatures, iiNet Toolbox, e-mail and our corporate websites are unavailable. Apologies for any inconvenience caused," iiNet tweeted.