The guns used in this terror attack are difficult to find and extremely expensive.
They're already illegal here.
The gun laws either had no impact, or prevented someone nearby who otherwise would have owned a gun
Discussion
They're not illegal at present (go on the Beretta Australia website), but it is true that you can't prevent people from owning guns because you can't prevent them producing them themselves. Bikies have been caught with completely homemade automatic weapons.
Even without 3D printers, smithing of firearms by sole traders was possible in the 19th century