Stijn
There was a what I would describe as a "Generous Buy Back" as part of the law changes and a lot of people took advantage of that.
And just quietly I have it on good authority that not all of the guns were handed in.
I handed mine in because I didn't want to jump through the Gumbits hoops to keep them LEGALLY.
Here's a link to my state's guns laws if you're interested - Excellent reading if you're ever having issue sleeping...
You'll note that sling shots (home made and commercial) are now illegal to posses in South Australia. This is a recent addition.
https://www.police.sa.gov.au/services-and-events/firearms-and-weapons