Firefighters say an explosion was heard at a dairy factory in Waikato and a witness described seeing workers sprinting from a building after a "deep boom" rang out.
The boss of Open Country Dairy, however, said the incident was a fire and disputed there was any explosion.
The alarm was raised at 9.43am when Fire and Emergency New Zealand was alerted to apparent sounds of an explosion and a smell of gas at the site in Horotiu.
Firefighters on scene found no evidence of a fire but confirmed an explosion.
Open Country chairman Laurie Margrain later told the Herald there was a fire but no explosion at the factory. He said the fire was a result of a fault in an air heater.
Margrain said there was no injuries and he did not think the incident was a major one but that it may cause a production hiccup
Police are also in attendance but firefighters are running the response operation.
A man working near the dairy factory said there was a massive boom before plumes of smoke or steam poured out.
"I heard a deep boom," said the eye-witness.
"Then I saw all this smoke or steam coming out of the building," he said.
He said workers sprinted out of the plant soon after.
He said the explosion was so huge he heard the blast through earmuffs and the sound of a chainsaw.
He said he could see a massive vent on the side of the building that had been blown out by the force of the blast. There were also black scorch marks visible at the bottom.
But there were no flames, just "heaps of smoke".
"We stopped work and looked over at the time. Everyone was running around and the alarms started."
Fire northern communications manager Craig Dally said the building had been evacuated, there were no injuries and the area was being contained while firefighters awaited the arrival of the gas authority to switch off the gas.
The incident would be investigated after that.
Open Country is New Zealand's second-largest dairy company. Earlier this month it was convicted and fined $137,500 for discharging odour that caused nausea, retching and vomiting for some in the Waharoa community near Matamata last year.
The company was also ordered in the Hamilton District Court to put $120,000 in a trust for the community as reparation.
It was the fifth environmental prosecution against Open Country in the Waikato.