A FAST moving bushfire is threatening a town in the Grampians National Park in Victoria's southeast.

The Country Fire Authority says residents in the town of Mirranatwa, west of Melbourne, have been told the 300hafire is expected to impact their town by about 5.30pm today.

An emergency warning has been issued for the blaze which broke out on Thursday following lightning strikes.

A CFA spokeswoman says crews are 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."