Firefighters were called to Fighters Xpress gym in Dandenong after receiving multiple calls reporting a structure fire 3.45am.
The gym was fully involved in flames when crews arrived on the scene, a Fire Rescue Victoria spokesperson said.
Fire crews have extinguished the blaze and the incident has been handed over to Victoria Police.
A Victoria Police spokesperson said a car was used to crash through the front door of the gym shortly after 3.30am.
“The offenders have poured accelerant through the building before setting it on fire and fleeing,” the spokesperson said.
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“A short time later, emergency services were notified of a vehicle fire on Canberra Avenue in Dandenong which is believed linked.”
FRV Paul Foster said the building was “quite severely damaged”.
“The structural integrity has been compromised. We managed to cut the fire off before it reached other adjoining buildings,” he said.
The fire has been deemed suspicious.