Reports I've seen say many Australian bushfires are either arson, or suspected arson started.
It is certainly true that firebugs start a lot of fires across the nation and it happens every year. I don't know if the fires started by firebugs are believed to proportionally higher this season. I don't think firebugs are an exclusively Australian disease.
From what I recall there was a study a couple of years ago by the Institute of Criminology (I think ) that said about 15% of bushfires were known to be arson with about 35% suspicious. IFIRC, about 10-15% were caused by natural causes, including reignition/spot fires, with just over a 1/3 were caused accidentally (hot exhausts, sparks from grinding/welding, back-burning, campfires, etc).
Caution was advised about the results because just under half of fires did not have an clearly identified cause, but there is no doubt in the report that humans intentionally or unintentionally, caused most bushfires.
Mark