The fine was severe because the offender was charged under the Criminal Code, not the Traffic Act - specifically Chapter XXVII, section 266 - which clearly outlines that anyone in charge of a "dangerous thing" - whether living or inanimate, and whether moving or stationary - and which definition includes a source of ignition or fire - is held totally responsible for any act endangering the health and safety of anyone near the "dangerous thing".
CRIMINAL CODE ACT COMPILATION ACT 1913 - NOTES
If he had been charged under the Traffic Act with a traffic offence, or offences, the penalty/penalties would have been less.
However, he wasn't driving an unroadworthy, road-going vehicle - he was driving an aircraft on a public road.
IMHO, the Police and the Magistrate have viewed his stupid act with the gravity that is applied to serious aviation offences and criminal offences, and have applied the appropriate penalty.
If you view his offence as a trivial one and the penalty as being too severe, then you fail to understand the gravity with which aviation offences are viewed in Australia.