It takes one exceptionally dedicated team of super-arsonists to burn a million hectares of cold wet forest. Such a thing has never occurred in history.

Weather patterns (hot and dry summer) create the conditions for a fire, an arsonist merely provides the ignition source. In the right conditions a single errant spark from a power tool can set off a giant bushfire. Or an arsonist with a single match.

Do you have a reliable source for the claim that “many of today’s Australia fires are arson”?

Well as I looked it up all of today's headlines are refuting prior days headlines about arson. So now I'm synced up.

These headlines were only ever in dodgy conservative media sources (generally conflating people being warned for campfires, BBQs and angle grinders outside with "arsonists deliberately setting fires"). You should check the ownership of the news sources you previously used and stop reading them.

  24 people have been charged over alleged deliberately-lit bushfires
  53 people have had legal actions for allegedly failing to comply with a total fire ban, and
  47 people have had legal actions for allegedly discarding a lighted cigarette or match on land.

https://www.police.nsw.gov.au/news/news_article?sq_content_s...

Source: NSW police official website

That sounds about right - but the bits of the media with a vested interest in portraying this as an unusual arson wave have been saying 180+ people arrested for lighting bushfires.

This police report only notes 24 arrests for alleged arson, not which fires were set, how big they got or whether this was unusual. It could well be that this is a typical (or small or large) arrest haul and that the fires involved were easily controlled or small. What we can tell with certainty is that NSW is only one state and so this report doesn't say anything at all about fire ignition sources in the other states: WA, NT, Vic, SA, Qld.

A reasonable assumption might be that fires in unpopulated areas in central Australia were probably not arson (no people there to set them) and these are the largest fires (no people there to fight them).

But rather early for any broad conclusions. All we can say is - there's been some arson but we lack the context or details to really understand the relative importance. Currently arson doesn't seem like a major factor and it is probably over represented in densely populated NSW relative to the rest of the country.

I would say there's little evidence to support a hypothesis of an "unusual arson wave".

That's right.

The authorities (don't know if this is RFS or police in this case) seem to pretty confidently attribute fires to causes for the most part. The percentage attributed to arson is lowish but disturbing (20%?).

I imagine lightning strikes and ember spread from existing fires are fairly easy to diagnose. I'm not sure how you tell "fires spread from sparks from roadway incidents" from "arson" but I'm not an expert.