Australian authorities urged people in dozens of rural communities to leave home "immediately" on Thursday to escape an out-of-control bushfire tearing through a national park.

About 600 firefighters were battling the blaze in the Grampians National Park 240 kilometres west of Melbourne, a Victoria state emergency services spokesperson said.

The blaze has persisted for more than a week in hot, windy conditions, scorching 55,000 hectares (136,000 acres) – about one-third of the park – so far without causing any deaths or destroying homes.

State emergency services warned residents to leave home immediately in more than two dozen mostly small rural communities, with populations ranging from as few as six to as many as several hundred.

People in several other communities were told to take shelter indoors because it was unsafe to leave.

Firefighters expected shifting winds to complicate their task during the day, said Victoria state control centre spokesman Luke Hegarty.

"We are reaching a critical part of the day when we see the wind change moving through the western part of the state," he said in an afternoon update.

"We're expecting strong winds and variable winds to be a concern for us over the next few hours."

A total fire ban was declared across the whole of Victoria, barring any fires in the open air. (AFP)