Here is a good piece poking fun at Australian prohibited weapons law:
http://gunwatch.blogspot.com.au/2017/08/australian-gun-culture-part-15.html
The laws relate to weapons that are not firearms. For example, Biblical era missile launchers, potato and water balloon launchers are banned. No crossbows, no slingshots as used by kids a few decades ago, all types of martial arts weapons (anything found in a Bruce Lee movie), no personal alarms, and the list goes on and on.
But, to simplify, in America, which holds to the English common law position, now eroded in England and Australia, everything is allowed except that which is prohibited. But, in Australia, everything is prohibited, except what is allowed. As with every other issue, these erosions of freedoms have been allowed to occur over many decades because the masses of ordinary sheeple people just don’t take a stand against them. We have slid into being a totalitarian police state, and this is only early days yet. As the song says, they have only just begun:
https://www.youtube.com/watch?v=__VQX2Xn7tI