I’m guessing that grassfires in the uninhabited Centre and North, which are a regular occurrence, don’t get listed by the Forestry Commission as BUSHFIRES.
The graph appears to have included evidence of lightning-strike fires as picked up after the event by satellites.
https://en.wikipedia.org/wiki/Bushfires_in_Australia#Major_bushfires_in_Australia
So much for post mortem forensic brush fire analysis.
I do not believe Australia has had such a serious situation before. The old timers would have left us stories and songs about it.