You have to register them. It is not difficult if you belong to a gun or hunting club.
https://www.ssaa.org.au You are correct that a lot of people gave them up, but these were not serious people who wanted a gun. You have to remember that Australia's population is mainly in the cities and these people, for the most part, have never touched a gun. Most rural families, especially on the farms still have legal guns. So now there are actually more guns in Australia than before the restrictions were put into place.