You didn't come near any of them.

Yes, there have been mass shootings in Australia since 1996, by the definition you presented here. The Salon article links to a number of papers. Not all the links work, but the papers are readily findable. The Chapman/Alpers paper (also cited in other papers in the article) defines "mass shooting" as five or more dead. Which definition are we going to be using here? Pick one, and run with it.

It's not uncommon for articles to try to make a case by cascading links to many different authorities, but often when you follow them up, particularly when the authorities are other articles, you find that most of them are based on the same original academic sources. In this case, almost all of these end up back with Leigh and Neill's paper. I call this "fulcrum science," and that's another story. But if you want to look at the fulcrum, check out Figure 1 in that paper. These are rates, not straight numbers, but correcting for population increase just changes the overall slope, not the shape of it (assuming increase is relatively smooth over those years).

You see any major changes in the overall curves? Side note: that peak in '96 works out to be basically the Port Arthur massacre (I calculated the increase of 34 victims to raise the rate/100,000 by about .2, assuming my decimals held up).

No, the rate of decrease did not "double" except for that one 96-97 outlier, whereupon the slope flattened right out again. You don't get to pick the right side of a peak and say what you did right after a massacre reduced the murder rate.

The real question you need to ask is what happened around 1986 to both gun suicide and homicide rates?

The figures are right in front of you, even from the most quoted sources and you give me "argument from authority?"