Sydney  - To some, it's just fun. To others, it's psychopathic behaviour they can't control. To Australian Prime Minister Kevin Rudd, deliberately starting fires on scorching days in the wooded fringes of towns is nothing short of mass murder.

"There are no other words to describe it other than mass murder," Rudd said, after the death toll in the nation's worst-ever forest fires topped 100.

A report last month from the Australian Institute of Criminology stated that around half of forest fires are deliberately lit, in what is largely an Australian phenomenon.

The report's author, Damon Muller, explained that few arsonists are caught. The average punishment, if convicted, is a year in jail.

Rebekah Doley, who specializes in the psychology of serial arsonists and who has interviewed 130 of them, says arson is a simple crime but perpetrators are complex.

"They also tend to be serial offenders and, in my experience, will not stop until caught," she said.

Which means that those who lit this weekend's fires, and learned that their handiwork had incinerated whole families inside their cars, will likely be at it again when the temperature rises and the winds pick up.

Kieren Walshe, deputy police chief in Victoria where 400 fires were burning on Saturday, doesn't doubt that arsonists are at work.

"These things have to have some sort of human intervention," he said. "They can't start from the natural elements."

One suggestion is to have a national register of convicted, or even suspected, arsonists and to vote through legislation that would allow them to be kept in custody on days like last Saturday when the temperature in Melbourne was 46 degrees Celsius and high winds were blowing though tinderbox forests. (dpa)