The Swan Hill CFA captain has urged farmers to remain vigilant around hay stacks after volunteers responded to a spate of blazes in the past month.
The fires come shortly after the new season's baling.
"They're starting by spontaneous combustion, in the middle of a paddock," CFA captain Dennis Turner said.
Recent rainfalls have wet uncovered bales and when this moisture and other nutrients break down under the hot sun a reaction can occur.
Cpt Turner said burning bales are left to their own course because adding water prolongs the smouldering.
It took more than 50 firefighters 10 hours to contain a similar hay blaze in the early hours of Christmas Eve at a piggery in Macorna.
A shed containing more than 1000 bales ignited.
Macorna CFA captain Daniel Crichton said it was fortunate the pig enclosures 100 metres away were not affected, with wind blowing flames towards another hay shed 30 metres from the blaze their main concern.
The following day, the Swan Hill Fire Brigade was called to another haystack fire on a Lalbert Lake Road property near Lalbert on Christmas Day.
Volunteers again responded after more than 2100 bales erupted in flames at a Mincha West property early last Thursday.
"Farmers need to now be going around and checking all their hay stacks and keep monitoring them," Cpt Turner said.
"My advice is don't stack them too close together and stack them in different locations around your farm and limit the size of the stacks.
"A crow bar can be stuck into a bale and if it's hot inside the heat will travel along the bar by conduction.
"If the bale is hot in the middle, you will feel the heat on the bar outside the bale."
Meanwhile, Swan Hill CFA was joined by six other brigades in tackling a grass fire on a Grey Road property in Meatian West, 43km southwest of Swan Hill, last Wednesday.
Cpt Turner said it's believed a truck in a paddock sparked the fire, which the owners began extinguishing before firefighters arrived about 11am.
It took crews from Berriwillock, Goschen, Lake Boga, Lalbert, Ultima and Woorinen South half-anhour to bring under control.
"The fire hit a tree line which slowed the fire down and gave us the chance to jump on it," Cpt Turner said.
"We put an Advice warning up to inform locals that a fire had started and to monitor the conditions."
For tips on how to prevent haystack fires, connect at www.cfa.vic. gov.au/plan-prepare/hay-and-firesafety/