Couple major differences. There weren't already 300 million plus guns in Australia and Australia is an island. The cost on the black market for the limited guns left sold by people who didn't turn them in are astronomical. As Jim Jeffries says if you have enough money to buy a gun on the black market in Australia you're a good little saver, keep going.
Also with easily passable borders for America guns would still easily be available to the criminals that would want to use them. Take cocaine for example it's 300 a gram in Australia and a fraction of that in the US.. both equally illegal but far more rampant in the US because it's cheaper and easier to get through tunnels, cars, boats, butt holes, etc.
I'm all for common sense gun laws, but whenever I see the Australia situation compared to the US it lacks consideration of far too many variables.