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.