Fire and Emergency NZ crews were called to the blaze at The Bridge in Springs Rd, Prebbleton, just after 5am today.
Crews from the Lincoln and Wigram stations took about 40 minutes to bring it under control.
They were so hot they combusted, he said.
"At this stage, we are looking at it being spontaneous combustion."
Jeffery said the towels had been left in a bunch and the heat within them was enough to cause them to catch fire.
"They were sitting there bunched up retaining all that heat so there still a bit of oil residue in and eventually, over time, it has combusted."
The Bridge co-owner Melissa Smith said the fire was upsetting.
"We are devastated but it’s contained and thankfully nobody hurt."
The fire damaged the kitchen and they should be able to reopen the bar and eatery soon, she said.
Springs Rd, the main route through Prebbleton, was closed to all traffic while crews worked on the fire.
The road reopened earlier this morning.
The Bridge opened last year after new owners Melissa and Murray Smith took over the lease for the building in August.
It was previously home to an Armadillos restaurant, which closed in 2022.