Ok there's some variance between your 'hundreds of environmentalists were caught' and your '24 were caught' In the article you offered as proof for your 'hundreds'.
Police contradict claims spread online exaggerating arson's role in Australian bushfires | Australia news | The Guardian
Here in Northern California I've never heard of arson as a political statement. Arson arrests seem to be folks who want to work the fire lines so they go out and start something, or in a few cases might be revenge or are just plain crazy. (And accidental fires plus natural causes - lightning etc - far outnumber intentional arson). Anybody with good sense environmentalist or not recognizes the lack of minor fires for a century here has led to what is now explosive growth of any fire that gets started, so wouldn't intentionally start one. Again what source do you cite as your basis for 'hundreds were caught'?