The OP is wildy going beyond data and one should have data skills, I would think:
If the laws were passed 22 years ago and it took that long to get a rather small but tragic shooting, how can one conclude the law is a failure? Not that I'm defending their laws but nonsensical conclusions from this one incident is the kind of rhetoric that hurts the RKBA cause.
In a surface validity argument, you would say:
The antigunner would say:
We don't have such laws and we've had since 1996:
Location, year, death, injuries, gun types.
Las Vegas shooting 2017 59 (inc. the
perp.) 851 Semi-automatic rifles
[50][51]
Orlando nightclub shooting 2016 50 (inc. the perp.) 58 Semi-automatic rifle and pistol
[50][51]
Virginia Tech shooting 2007 33 (inc. the perp.) 23 Semi-automatic pistols
[50]
Sandy Hook Elementary School shooting 2012 28 (inc. the perp.) 2 Semi-automatic rifle and pistol
[50]
Sutherland Springs church shooting 2017 27 (inc. the perp.)
[nb 1] 20 Semi-automatic rifle
[52][51]
Marjory Stoneman Douglas High School shooting 2018 17 17 Semi-automatic rifle
[53]
San Bernardino attack 2015 16 (inc. both perps.) 24 Semi-automatic rifles
[50][51]
Columbine High School massacre 1999 15 (inc. both perps.) 24 Multiple types of firearms
[54]
Binghamton shootings 2009 14 (inc. the perp.) 4 Semi-automatic pistols
[54]
Fort Hood shooting 2009 14
[nb 4] 33 (inc. the perp.) Semi-automatic pistols
[54]
Washington Navy Yard shooting 2013 13 (inc. the perp.) 8 Semi-automatic pistol and shotgun
[54]
Aurora shooting 2012 12 70 Multiple types of firearms
[54][51]
Geneva County massacre 2009 11 (inc. the perp.) 6 Multiple types of firearms
[54]
Atlanta shootings 1999 10 (inc. the perp.) 13 Semi-automatic pistols and revolver
[50]
Red Lake shootings 2005 10 (inc. the perp.) 5 Semi-automatic pistols and shotgun
[54]
Umpqua Community College shooting 2015 10 (inc. the perp.) 8 Semi-automatic pistols and revolver
[54]
That's not counting failed attempts and the general shootings in the urban areas that are in the hundreds. There are others that weren't in the list I copied from Wikipedia, such as
Thus, the OP would not be convincing in an argument. If you want to argue Australia, that's not the way to do it.
As PhD in psychology, I would advise folks about the inoculation effect. If you challenge someone with a weak argument, you end up strenghtening their beliefs. So if I were on stage with the OP who stated that the Australian laws failed because after 22 years of their laws since a massacre, they had a domestic shooting - I would just reel off the USA incidents. Anyone who had antigun beliefs would have them strengthened by the OPs assertions that:
Their laws would look pretty good. Try again.