It is understood about 12,000 bales and two hay sheds, worth more than $1.5 million, were lost in the fire which broke out at Quigley Contracting near Ashburton on May 14.
It is the second such fire at the site; there was a similar blaze in April 2021.
Fire and Emergency NZ could not say what the cause was when asked by The Ashburton Courier on Tuesday, as an investigation was still under way.
But Andrew Quigley from Quigley Contracting said while the cause had not yet been confirmed, it was probably an "unfit bale".
Internal combustion, or spontaneous combustion, in a hay bale occurs when moisture and organic material within it cause a chemical reaction that generates heat.
The bales were high-density and, if a bale was also high moisture, it could have combusted.
"We have pretty strict procedures and processes around not letting that happen," Quigley said.
"Probably the most gutting thing from our point of view, this has happened again four years later, it’s absolutely devastating," Quigley said.
"A lot of the product was presold all over New Zealand, it’s a big disruption to our business."
One of the two sheds that burned down was under construction at the time of the 2021 fire, the second was about 10-years-old.
Quigley said the fire was first noticed by a neighbour.
Quigley was grateful to the "incredible" and "tireless" teams of firefighters. Brigades from throughout Canterbury were called to the fire.
He was also grateful to Stuart Tarbotton Contractors and Harmer Earthmoving which helped with the clean up.
