From what I understand, teh arson figure is overblown but there has been a few attempts to start more fires or successful attempts to do so. This would make sense cause its common in fire zones (California has this issue as well when fires start you find a few people who like to start fires under the cover of already occuring fires). So yes, some of it is arson.

As for global warming...That one is a bit trickier. Some places are reporting that on average there has been more rainfall in australia for the past 20 some odd years, not less. This doesnt mean things arent going through a dry spell currently. The biggest point I seen being made is activist groups who are pro environmentalism seeking (and being successful) in reducing controlled burns prior to this. This is a problem if you do live in places where dry brush and other combustibles can gather. Again pointing to California, a lot of the bigger fires that have happened in the past 10-20 years have been because the state stopped doing controlled burns and clearing brush under progressive environmental legislation and decision making at a local level. So when a fire does occur, it gets out of control very quickly.

If there is global warming involved in this, it is more likely to be something exacerbating the situation rather than being the root of it. Warmer weather or dryer climate does not mean that wildfires come into existence out of nothing. It just, at best, would speed up the collection of dry brush and other flammable materials in fire zones at a faster rate than before. If you pair this concept up with higher rainfall prior to this excessive dry season, it would make more sense (Higher rainfall = greater plant growth = more dead brush and shrub when dry season hits). So looking at it pragmatically, Global Warming or Not, the Australian government (both local and federal) should be watching the weather and local growth a lot more closely and doing controlled burns and clearing out dead shrubs to keep the amount of flammable materials to a minimum and mitigate against large fires.

Point is theres a lot more nuance (like usual) to whats happening than "Oh global warming is the cause of this!".