NSW RFS Riverina zone operational officer Bradley Stewart said 11 haystack fires were recorded across the region since October, 2019.

"Those occurred in places like Junee, Coolamon, Lockhart and Wagga,with the most recent as late as Thursday near Henty," he said.

Mr Stewart said the risk was "unfortunate" for farmers producing hay.

"If there is even 1 per cent too much moisture in the hay, bacteria can get in, it starts to heat up and can get to the point of spontaneous combustion," he said.

NSW RFS Inspector Doug Adamson said farmers should check their haystacks on a regular basis by breaking them open or by inserting a crowbar or metal rod to assess temperature.

Farmers should also ensure any valuable stock and machinery is stored separately.