You cannot eradicate hate from the minds of those who are determined to hate someone, anyone, anything with a piece of legislation.
You already had laws against shooting people. What good did they do?
No damned good at all.
Do any Australians, for even a minute, think that passing a law will stop people from killing other people?
In America, numerous such events, both large, and intimately small, have been stopped by armed citizens, carrying their weapon of choice. Please take note of that. It has been estimated that potential victims being armed has stopped over 2 million crimes from being committed every year for some time.
While opinions may differ in political discourse, the harsh reality is that even those who choose not to have arms (and brazenly advocate against other citizens having them), depend themselves on those who are armed to keep them safe from criminals and terrorists. They hate the idea that ordinary people might have the same protection, provided by themselves.
In America, we still have the option, actually, the Right to choose to defend ourselves, others, and if need be, our community, from those who would so us harm (in violation of all those other laws which preclude doing us harm).
You had no such. No armed citizens--even the man who disarmed the terrorist did not take that weapon and turn it on the terrorists, so heavily have you ingrained the notion of not defending self and community.
Where are the men who stood against the Japanese, who fought in Korea, in Vietnam? Mostly, if not all, dead, as is the fundamental right to defend one's self with the best tools available, denied your population because of the acts of an insane few, who, incidentally, still hate you.
And you continue to pursue a policy which has clearly failed.
When you have destroyed all the guns that were privately held (which have not been hidden away in violation of your sensibilities and laws), who will defend you when the Chinese invade, or some Islamic cleric gets a notion to do away with the kaffirs? Just 80 years ago, a hostile Army (and Navy) was bearing down on you, but Australia was better armed, and Australians were made of sterner stuff.