Permalite, Australia has 18M people back then. 18M v 323M. It's common sense that it's nowhere near the same. Australia is massive landwise but the majority of people are concentrated in the capital cities or around them.
And the government only had about 1M that were surrendered or bought back. Imagine trying to buy back 200M guns.
And we know that criminals can get guns in Australia - we've seen it multiple times over the past years.
And for the record - the surrendering of the firearms in australia wasn't voluntary - the government spent around $500M to buy back 660,000 guns.