Woolsthorpe fire brigade captain Simon Cousins said the bales caught alight on a Woolsthorpe-Heywood Road farm about 4am.

No one was injured.

About 30 crew members from Woolsthorpe and surrounding brigades attended. A bulk water tanker also assisted.

"There was another stack of hay further down along the trees that weren't impacted," Mr Cousins said.

He said the bales were stacked in a row under Cyprus trees, and the fire spread to the paddock.

The CFA captain said the blaze was difficult to put out, and crews placed a containment line around it and spread out the hay.

"We're going to let it try and burn out and get away the trees," he said.

The Country Fire Authority is investigating the cause of the blaze.

Smoke may be in the area the next two days as the bales continue burning off.

Vic Emergency said there was no threat to the community. Emergency services remain in the area.

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