The first result on google for 'Australia gun ownership rates':

https://www.sydney.edu.au/news-opinion/news/2021/04/28/new-gun-ownership-figures-revealed-25-years-on-from-port-arthur.html

-Australian civilians now own more than 3.5 million registered firearms, an average of four for each licensed gun owner.

-The proportion of Australians who hold a gun licence has fallen by 48 percent since 1997.

-The proportion of Australian households with a firearm has fallen by 75 percent in recent decades.

-Data indicates that people who already own guns have bought more rather than an increase in new gun owners.

And I don't know much about their mass shooting history, but here's an article explaining that homicides and suicides sharply declined after the ban:

https://www.vox.com/2015/8/27/9212725/australia-buyback

What they found is a decline in both suicide and homicide rates after the NFA. The average firearm suicide rate in Australia in the seven years after the bill declined by 57 percent compared with the seven years prior. The average firearm homicide rate went down by about 42 percent.