Not again... why won't this myth die?
This idea that guns are banned in Australia has wide currency in the United States, but it simply is not true. There are millions of firearms legally held by private citizens in Australia - more, in fact, now than before the supposed ban (although it did take about fifteen years for numbers to rise again to that level). We've certainly got some dumb gun laws in Australia - universal registration, certain restrictions on things like handgun barrel lengths that make no real sense, and strict rules on things like storage, but guns aren't banned.
I'm an Australian gun owner myself. I own both a handgun and a semi-automatic military-surplus rifle, and could own more if I wanted to (I don't see the need myself as for me firearms are tools - I can only use one gun at a time so what I have meets my needs - if I was interested in collecting, I'd own more). I do own a few basic hunting rifles as well, but am considering rationalising those down to one and getting a decent shotgun as well.
These criminals generally don't follow the laws that we do have - most of them would not be legally allowed to own firearms because of serious and/or recent criminal convictions - so the incidents do illustrate that laws certainly don't stop criminals getting guns. But the laws also don't stop law abiding citizens getting guns either - and a significant proportion of the adult population do, to at least some level (basic hunting rifles and shotguns probably account for at least half to three quarters of all guns, because owning more than those requires navigating a substantial bureaucracy - as I say, dumb laws).