Fire and Emergency NZ (Fenz) data released under the Official Information Act showed spontaneous combustion caused at least 114 fires last year, up from 91 the year before.
Sutherland said three different investigations were carried out after the blaze erupted 70km north of downtown Auckland.
It was found that the fire erupted from a laundry bag filled with tea towels which had at some point soaked up organic oil.
"It's quite freakish," Sutherland said. "You do the same thing every day for four years. Everything's fine and then..."
Video footage showed laundry being returned to the Sawmill.
A few hours later, the pile started smouldering before flames quickly ripped through the building.
"It's quite fascinating it can happen for starters. The results are fairly catastrophic," Sutherland said.
He said a fire investigator grabbed a scrap of cloth in the wreckage after the fire and told him to sniff it. It was a familiar, unmistakable smell.
"Basically the cause is the residual cooking oil."
Lucy Cotterill, of Fenz, said spontaneous ignition was a chemical or biological process generating enough heat to ignite reacting materials without introduced heat sources.
"Common instances of spontaneous ignition include fires in hay piles, compost piles and fabric that has been used to apply or soak up organic oil," the Fenz chief adviser said.
Other hospitality businesses have also learned about spontaneous combustion the hard way.
From Oamaru to Auckland, 14 different breweries helped Sutherland and co-owner Kirsty McKay get the business running again.
And after surviving the extra challenge of the Auckland August coronavirus lockdown, Sutherland said he was looking forward to a busy and successful summer.