The most recent detailed statistics I could find are from the ABS Australian Yearbook 2004, and are for public land in Victoria, but they're interesting. They show the percentage of burned land due to each cause.
46% Lightning
14% Deliberate
14% Public utilities (e.g. electricity cables sparking)
9% Building fires, fireworks, and other miscellaneous
7% Agricultural
5% Prescribed burn escapes
3% Cause unknown
2% Machinery/exhausts
1% Campfires
<1% Cigarettes/matches
That 1% for "Campfires" will be mostly picnickers and car campers; bushwalkers will be responsible for a vanishingly small amount