By Matt Siegel and Jane Wardell

SYDNEY, Jan 6 (Reuters) - Hundreds of firefighters were battling on Tuesday to contain Australia's worst wildfires in 30 years which have already swept across more than 12,000 hectares outside the southern city of Adelaide and destroyed at least 26 homes.

The fires, which are burning across a 240-km (150-mile) perimeter in the state of South Australia, come as the Australian Bureau of Meteorology announced on Tuesday that 2014 had been the country's third warmest year on record.

That has raised questions about whether the blaze is the result of climate change and a possible sign of worse to come.

"Unless there are rapid, substantial and sustained reductions of greenhouse gas emissions in Australia and globally, Australia will experience more heat waves and bush fires as in 2014," David Karoly, a professor of Atmospheric Science at the University of Melbourne, said in a statement.

At least 29 people have been injured or taken to hospital but no deaths have been reported.

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 next 48 hours were critical to firefighting efforts because of higher forecast temperatures and changing winds.

"Today we will see easterly winds coming through and by mid-afternoon, 4 p.m., we're going to be back to 36 degrees Celsius (96.8 degrees Fahrenheit)," Eden said. "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 some scientists say climate change is increasing both the fire season and intensity.

The heat caused different problems in the West Australian capital of Perth where thousands of iiNet Ltd 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 degrees Celsius on Monday, the hottest January day since 1991 when it reached 45.8 degrees.

"Due to record breaking temperatures, iiNet Toolbox, Email and our corporate websites are unavailable. Apologies for any inconvenience caused," iiNet tweeted. (Reporting by Matt Siegel and Jane Wardell; Editing by Nick Macfie)