"It was that hot," the owner said.
"The whole building looks like someone has painted it black.
"An alarm 20m away from the source of the fire is a molten mess."
Fire services arrived about 6.15am to assist the men, who were taken to hospital to be treated for burns and smoke inhalation.
The tenant who burned his hand and elbow also burned his feet when he walked on hot concrete inside the building.
"The damage that was done tells me the fire must have been going for a while," the owner of the building said.
He said the tenant used the building mainly as a workshop, storing supplies for his painting business.
Most of the paint in the building was non-flammable, and they had insurance on the building and contents, he said.
He said it amazed him the number of people who did not have insurance and this proved that people should be prepared.
The cause of the fire is being investigated by the Rotorua Fire Service, who do not think it is suspicious.
Meanwhile, there was a fire at Kinleith's Carter Holt Harvey Pulp and Paper Mill yesterday morning.
The mill's own firefighting team called in extra help from fire services at Tokoroa about 4am.
However, the fire was eventually contained inside a gas boiler and the fire service was stood down.
No one was injured in the incident.
Pumicelands Rural Fire Authority principal rural fire officer Paul Wright said the fire danger in the area was steadily increasing, because of dry conditions.
Firefighters had worked through the weekend dampening down hot spots on Waiotapu Loop Rd, near Wai-O-Tapu Thermal Wonderland, after a big fire struck on Friday afternoon, he said.
A cause is still to be found for the fire.
Mr Wright said vegetation across the entire central North Island was extremely dry and there was no significant rain expected for at least another two weeks.
Mr Wright said they had someone stationed on the top of Rainbow Mountain each day now, to keep a lookout for potential fires.
He said the general public had been good at staying vigilant and respecting the total fire ban.