As a local effected by the gun buy back, I will give my recollection though it is fading after 25+ years.
I have a small farm so had a Ruger 10/20 and an air rifle. After the Port arthur incident the whole licensing issue was tightened and the buy back instigated. Prior to the event each state had its own regulations. West Australia (my home) had the strictest. Each firearm is registered to a person to use. I cannot lent my rifle to another licensed person unless the particular rifle is listed on their license. It is slightly different for farmers as the farmer can let a worker use the rifle for farm purposes.
The other states had much less strict rules that allowed that a person simply got a shooters license and could hold and use what ever they wanted. This meant that no one knew what firearms where around.
After the incident, the rules were unified to those like WA with an addition that firearms needed to be stored in a safe.
At the same time there was a buyback scheme put in place, This allowed anyone to hand in any type of firearm, no questions asked. I think the amount was fixed (I think there may have been a claim for more for a more valuable firearm). There were no doors kicked in. The classes of licenses allowed different types of firearm. Automatics are forbidden. Semi automatics are permitted for primary industry purposes. If you had a semi auto and no farm, you had to hand it in.
It is still possible to get a license as long as you have somewhere that you allowed to shoot on. Firearm licenses are not issued for self defence purposes.
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