>The Australian Productivity Commission has calculated that between 2001–02 and 2006–07, [there was] an average of nearly 54,000 fires per year (SGRSP 2008). ... It is estimated that 50 percent of fires are either deliberately lit or suspicious in origin as shown in Figure 1.

https://aic.gov.au/publications/bfab/bfab059

edit: and to compare arson charges specifically, the same article says that between 2001 and 2006 there were 133 convicted bushfire arsonists in New South Wales - or about 20 per year convicted in a single state. Since there probably isn't a 100% conviction rate, I'd expect the total number charged each year for the whole country is along the order of 50 people.