Wikipedia has a list of fires, and it does not align with your chart.
https://en.wikipedia.org/wiki/Bushfires_in_Australia#Major_bushfires_in_Australia
It shows that the 1974/75 fires and the 2002 fires covered more acreage than the current fires.
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