Molotov cocktails are believed to have started three car fires in Tarneit in the early hours of Tuesday.
Police and CFA were called to Cottesloe Boulevard at 3.20am to find the cars set alight.
One vehicle received only slight damage as one of the devices failed to properly ignite. The other two were extensively damaged.
The fires were quickly extinguished and emergency services left the scene.
But at 5.20am the offenders returned to finish the job and the same three cars were again set alight and this time destroyed.
Police believe two offenders were involved in the first incident, driving to the location in a small two-door sedan.
On the second occasion, a man was seen standing next to one of the cars before it burst into flames. As he ran off, a woman chased him but was unable to catch up.
Anybody with information is being urged to call Crime Stoppers on 1800 333 000.
