It has promoted warnings to producers to take extra care.

Authorities say excess moisture at baling and heavy rain, soaking into stacks, are believed to have exacerbated conditions leading to spontaneous combustion of stored hay.

The CFA issued a warning for a haystack fire in Leichhardt, near Marong, last Friday, in a string of unrelated blazes stretching into northern Victoria.

District 22 Commander Peter Bell, Shepparton, said the issue was more prevalent this season, with three major fires in the area in recent weeks.

"Hay has been cut too green and stacked too tightly," Mr Bell said.

"The internal temperature in a stack can reach up to 55 degrees, then a chemical reaction begins and flammable gases can lead to ignition."

The CFA said it had responded to 50 hay fires, across the state, since November 1, 2023, with 42 since December 1.

In the north-east, CFA crews had attended 13 hay fires, since November 1.

Last financial year the CFA responded to 52 haystack related fires.

Mr Bell said it was important to limit the size of stacks and make sure they were not all in the one area.

"One farmer lost between 1000-1500 bales, because they were all stacked together - the moisture and temperature should be checked regularly," he said.

Hay must be kept dry and away from leaking rooves, spouts or runoff, he said.

"Cover the stacks with tarps, or hay caps," Mr Bell said.

"We always seem to get a few haystack fires a year, but this year it's been an exception.

"We have had rain then sunshine - it dries it out, heats it up, then you get more rain which is conducive to fire.

"It's all to do with the sugar content in the hay, because once that starts to crystallise you get spontaneous combustion."

Agriculture Victoria Dairy - North program manager Brett Davidson, said intense rainfall could compromise even well covered stacks.

Water could pool at the base of a stack, seep into gaps or could be blown under covers.

"Even at 2 - 3 per cent above the maximum moisture contents hay will start to lose dry matter (DM) and nutritive value due to plant respiration or mould growth, resulting in heating of the bale/stack," he said.

"Higher moisture contents, well above the maximum ( 4pc), could lead to more mould and heating, possibly resulting in spontaneous combustion."'

Mr Davidson said it was vitally important to regularly monitor the stack from one week after rainfall for signs of heating, including dampness on bale tops, steam, moisture build-up on roofing or unusual smells.

He said unfortunately, much of the heating occurred in the stack centre, which was difficult to pick up.

"To try and get an idea on how hot a stack has become, use a crowbar pushed into the stack as far as possible and after a couple of hours, remove the crowbar and feel how hot it is."

- With Tom O'Callaghan