The blazes - near Diggers Rest, Roxburgh Park and Craigieburn - occurred days before Crime Stoppers launched a campaign against deliberately lit fires.

The CFA attended a fire next to the Calder Freeway near Diggers Rest at 11pm on January 7.

The fire, which was north of the freeway, was put out quickly.

Constable Tristan Kassis, of Sunbury police, said the CFA had ruled out natural causes and deemed it was too far away from the freeway to be caused by a cigarette butt from a passing vehicle.

Crime Stoppers executive director Peter Sprott said 30-50 per cent of all fires were deliberately lit.

"Areas on the rural fringe are at greater risk and deemed to be fire-prone areas, with also a risk of arson and deliberately lit fires. With an increase in population, there's likely to be more people who fit the profile [of an arsonist]."

He said 36 people were charged with deliberately lighting fires in Victoria last year.

One person was charged with arson causing death. Penalties for arsonists include up to 25 years' jail.

Mr Sprott appealed for people overhearing "anyone dropping a hint" regarding arson on, say, Facebook or at a sporting club, to phone Crime Stoppers.

Meanwhile, police are investigating a fire at Bucklands Way, Diggers Rest, about 1pm on Sunday.

Tankers from more than 10 locations attended and the blaze was brought under control after about two hours.

Information to Crime Stoppers, 1800333000 or visit their website here.