The fire took 15 minutes to be contained by CFA crews after they were called out to the Edwards Road gym just after 3pm this afternoon.

Feelgood Fitness receptionist Jordyn Butler said about 20 people were in the gym when the fire ignited.

Flames and a thick plume of black smoke could be seen coming from the roof, with passersby alerting management to the fire at the back of the property.

“Everyone was pretty quick to get out and lucky everyone was quite relaxed about it,” Ms Butler said.

Bendigo Fire Station senior station manager Darren Eenjes said it was the third fire caused by an air conditioner in the past two weeks.