Strong by Blood gym co-owner Royden Rata said his physio alerted him around 7am that there was no power. As a result, the gym needed to be shut down, and he was unsure when it would be opened again.
He believed it was turned off, but he was unsure of why or for how long as no one had been in touch with him.
Rata said the blaze was about 50m from his building and he considered them "lucky" that it did not even have smoke damage.
He saw photos of it and said the fire was "pretty big".
The Rotorua Daily Post understands the fire was at the former site of the Carter Holt Harvey mill.
Crews came from Putāruru, Tokoroa, Tirau, Rotorua, Matamata, Ngongotahā and Tauranga, the spokesman said.
By 6.30am one crew remained at the site.
The spokesman said it was too early to tell whether the fire was suspicious.
Powerco confirmed it was alerted to a building fire on Princes St on Tuesday at 11pm.
To make the site safe, crews carried out a safety disconnection to the building, a spokesperson said.
"We're still waiting for confirmation on how many surrounding customers this has also impacted and when power is safe to be reconnected."
