Re figures, there is also
Australian Aircraft Activity with reports from 2002 to 2016 and some of those reports have historical data back to early 1990s. The data however is reliant upon the accuracy of reporting by operators and I think is more rubbery rather than rigorous.
FWIW I plotted what was in the 2016 report for 'other VH aircraft' from 1985 in 1000s of hours
The general trend seems downward but I think the numbers not highly accurate.
My hunch is better indication would be from avgas useage but it could be argued the rise of RAAus aircraft may not be accurately reflected here.
That also seems downward but as said - not necessarily a definitive reflection of activity.
My perception of decline is less objective but comes from the differences I see between when I was instructing in the 90s and now. At the same aerodrome.
In the 90s the aerodrome had 4 full time commercial flying schools, 3 other volunteer run training schools and an aero club.
Most of those non commercial schools/clubs operated mostly weekends but it was not uncommon for them to have ops during the week and were very active during school holidays.
There was a reasonable amount of private owner activity as well.
Since that time several of the commercial schools went broke, some were replaced by new ones but some of the replacements then also went broke. A nearby aerodrome was closed and one school transplanted itself here.
Now, there are only 2 full time schools left (one from back then and a new entrant that transplanted from the aerodrome that was closed), another one that operates part time mostly weekends (but sometimes remains closed even on weekends).
Two of the non commercial schools/volunteer run ones remain, one only operates weekends, the other still sometimes does a few days during the week. The aero club folded.
Most of the parking is filled with privately owned aircraft that rarely if ever move, grass growing up around them as they slowly sag lower and grow sadder looking.