Actually, it could rather easily be natural - Friday night, before the fires of Saturday - there were dry thunderstorms across wide areas of rural Victoria. Lots of lightning.
Some of the fires were deliberately lit. Forensic examination is indicating many of them were not. The possibility of some organised group lighting some of the deliberately lit fires can’t be eliminated, but there’s no automatic reason to assume it’s the case.
Bushfire is common in Australia. On any given day over summer, you expect 200 bushfires to be burning somewhere in Victoria. Most are small and in areas where they can be allowed to burn themselves out - but on a day with high winds and very high temperatures, things change a lot.
Alas, it seems the #1 cause is all too familiar to Californians—the big buildup of sacrosanct underbrush and deadwood and Mickey Mouse enviro regulations that wouldn’t allow locals to clear safe areas around their dwellings.
I read the article about how N.S.W. has done a much better job through reviving the old Aborigine methods of fire management in their areas.