Investigators say there is no apparent pattern to the fires. In one incident, a scrub fire was started after a lemon gun was fired in Cranbourne North. Another involved a molotov cocktail that was flung into a Narre Warren house.

Leading Senior Constable Lisa Christian of the Casey crime investigation unit said police were already investigating 19 other suspicious fires dating from the end of December before the weekend of January 28-29.

She said police were keen to learn more about the firing of a lemon gun or spud gun - a plastic tube filled with a flammable gas that is ignited to launch a projectile - in O'Tooles Road on January 29.

She said residents reported "a very loud explosion" coming from near the Cranbourne Soccer Club at 12.30am.

The explosion caused two small fires in the bushland near the soccer club, but they were extinguished by the CFA.

Residents said they had heard three similar explosions at half-hour intervals.

A dark-coloured large car, similar to a Holden Commodore, was seen driving slowly away from the soccer club after the fires.

Arson offences in Casey over the weekend included:

■ A molotov cocktail thrown through the front window of a house in Beverley Court, Narre Warren on January 29 at 4.10am.

■ A small scrub fire in Kirsty Lottkowitz Reserve at 11.30pm on January 28.

■ The rear door of a garage at a house set on fire in Raffindale Crescent, Cranbourne West, at 2.15am on January 29.

■ Cardboard boxes set on fire at the rear of a supermarket in Webb Street, Narre Warren, at 10.35pm on January 29.

A stolen Holden Commodore was torched behind sound barriers near the intersection of Princes Highway and Parkhill Drive, Berwick, at 7.05am on January 29.

A youth from Clarinda and a youth from Cranbourne North were charged last week with theft of a motor vehicle and criminal damage by fire over the burnt-out car.

They will appear in the Dandenong Children's Court on March 2.

Any information can be reported to Detective Sergeant Chris Lewis at Casey CIU, 97053114.