[benmmurphy]

I think the main sources are lightning strikes, accidental human ignition and deliberate arson. A lot of people have been arrested for deliberate arson recently (https://www.theaustralian.com.au/nation/nsw-firefronts-uncon...).

I would guess a combination of winds and high fuel load means a small fire can become an uncontrollable roaring blaze very quickly. I guess also a lot of fires could be started in areas that are not easily accessible to fire fighters so it might not even be an option to stop them while they are small.

[NeedMoreTea]

...and a fair few from the native Australian species, the Eucalyptus. Known to spontaneously combust and even spontaneously explode when spots of resin magnify sunlight. The leaves give off enough oils into the air that they can also spread a fire across a wide area through the tree tops. The plants and seeds are adapted to use fire as part of reproduction, and the seeds fire resistant. Just never normally on this scale.