https://en.wikipedia.org/wiki/List_of_massacres_in_Australia
A mass shooting in which, by the sound of it, a gunman killed one hostage? Then when "police...stormed the café ...[a hostage] was killed by a police bullet ricochet. ..[The gunman] was also killed. Three other hostages and a police officer were injured by police gunfire"
https://en.wikipedia.org/wiki/Lindt_Cafe_siege
In the 2002 Monash shooting, 2 people died. Seems like there hasn't been a mass shooting of >5 people in Australia since Port Arthur (1996), except in 2018 when a grandfather in WA killed his whole family including himself (7 people).
If you look at the numbers there over the years for actual shootings it's basically a bad weekend in Chicago.
That linked source is for all massacres, most of which are not gun related and have < 6 deaths. Even if you include the 2002 Monash University shootings, its still 20 years.
If you look at [1], when the Australian gun laws came into effect, a year later the per capita number of gun related deaths halved. 25 years later and its halved again and the trend continues downwards. For reference the US numbers are here [2]. What is interesting is that when comparing the number of firearm possession per capita between the US and Australia, the US has roughly 10 times more guns [3]. Based on [1] and [2] the US has roughly 10 times the number of gun related deaths per capita. The reason why Australia has historically had less problems than the US with gun violence is that even at Australia's peak, it had 5 times fewer guns [4] than the US [5] did per capita.
[1] https://www.gunpolicy.org/firearms/compareyears/10/rate_of_a...
[2] https://www.gunpolicy.org/firearms/compareyears/194/rate_of_...
[3] https://www.gunpolicy.org/firearms/compare/10/rate_of_civili...
[4] https://www.gunpolicy.org/firearms/compareyears/10/rate_of_c...
[5] https://www.gunpolicy.org/firearms/compareyears/194/rate_of_...
The Sydney hostage crisis only one person was killed by the gunman, who was carrying a shotgun. If he had an automatic weapon it very likely would have been more.
Your last point is true.