The group at the gym watched in horror as the roof was peeled off the house next to them and flung through the bedroom ranch slider of the elderly man in the house behind.
The windows of the gym started pushing inwards with three of them moving away and one staff member huddling under a bench, Ms Thomson said.
Luckily she did as a large window shattered leaving shards of glass all of the equipment, she said. "Thank goodness no one was in the way."
Ms Thomson said there was one person in the house whose roof was ripped off at the time.
"One of the girls was actually on the couch and she woke up and there was daylight," Ms Thomson said.
No one was believed to be injured as a result of the incident.
Two fire appliances from Mangonui were at the scene for just under two hours, a fire service spokesman said.
Metservice severe weather forecaster Erick Brenstrum said there had not been any recorded tornado activity in the region but it was not out of the question. "It could have been a small tornado or a straight line wind," Mr Brenstrum said.
To be categorised as a tornado there was typically a visible funnel cloud or the strong circular movement of wind, he said.
There was a weather watch in place for Northland overnight for periods of heavy rain and thunder. The reported mini-tornado came amid a front causing heavy rain and flooding in the lower North Island.
