Three water-bombing helicopters are battling a bushfire north-east of Melbourne, as the weather bureau warns Victoria's east of severe thunderstorms.

The slow-moving bushfire, which was sparked by a lightning strike about 11.30am on Monday, has burnt through 75 hectares of steep, rocky land near the town of Avenel.

"The fire has split into three fingers which are moving uphill towards a ridge line and continues to burn slowly in a south west direction," the Country Fire Authority said in a statement.

Twenty-one fire trucks and four helicopters are at the scene near Booroola station on Tarcombe Road.

Three water-bombing helicopters are being aided by a fourth helicopter which is monitoring the scene, a CFA spokeswoman said.