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A town in Victoria's southwest is under threat from a fast moving bushfire, the Country Fire Authority (CFA) says.

Residents in the town of Mirranatwa, west of Melbourne, have been told a 300 hectare fire burning in the Grampians National Park is expected to impact their town by about 5.30pm (AEDT) on Sunday.

The CFA has issued an emergency warning for the blaze, which was started by lightning on Thursday.

A spokeswoman said CFA crews were in the town working on asset protection.

"We've issued an emergency warning, it (the fire) may impact the town," The CFA spokeswoman said.

"We're expecting it to impact the town in the next two hours."