In August, a hay stack caught fire in Gumly that took days to stop smoking.

NSW RFS Inspector Doug Adamson said farmers should check their hay stacks on a regular basis and ensure any valuable stock and machinery is stored separately.

"Hay shed fires have the potential to cause considerable damage to farming infrastructure," he said.

"In some fires, the loss of hay bales and storage sheds has been extensive, but worse still these fires have also damaged or destroyed expensive farm machinery and supplies that were stored nearby."

Farmers should regularly inspect hay bales by breaking them open or by inserting a crowbar or metal rod to assess temperature.

"If the bar or metal rod comes out hot then there is a problem and the hay bale needs to be pulled apart as soon as possible to cool," Inspector Adamson said.

"There is considerable risk of hay igniting when it has been stacked while wet or not cured, so we recommend hay is stored in a dry, well-ventilated area away from other machinery at all times."