Electricity firms cut off power to thousands of people, more than 100 schools were closed and residents in high risk regions sought shelter today as Australia's devastating bushfires opened up a new front.

Australia has been battling wildfires across several states for days, endangering thousands of people in many communities.

Blazes so far this month have killed at least four people, burnt about 2.5 million acres of farmland and bush and destroyed more than 300 homes.

Today, a fresh battle line was drawn as 50 fires sprung up in South Australia state, where officials lifted the fire danger warning to "catastrophic" as temperatures passed 42C.

A catastrophic warning means that should a fire start, it will not be possible for firefighters to control it, given the weather conditions.

"From sunrise until well past midnight, this state is going to experience very difficult fire conditions," Brenton Eden, assistant chief officer at the South Australian Country Fire Service, told Australian Broadcasting Corp radio.

More than 600 firefighters attended to incidents across the state today and most were expected to keep battling fires throughout the night.

As some of fires approached electricity transmission lines, provider SA Power Networks cut power to over 12,000 customers.

With strong winds stoking blazes, authorities put residents near four of the fires on high alert to flee in case the flames spread rapidly.

"This is the worst of the weather from a fire behaviour point that we will have seen," Mr Eden told reporters in Adelaide, the state capital.

Australia is prone to bushfires in its dry, hot summers, but the recent series of fierce blazes have been sparked early, in the southern spring, after a three-year drought that has left much of the country tinder-dry.

While the immediate threat was in the south today, firefighters continued to battle about 100 fires that have been burning for several days across Australia's east coast.

Sydney, the country's most populous city with around five million residents, was covered with thick smoke for the second day running.

Health officials yesterday warned people in the harbour city to stay inside as the smoke reached hazardous levels.