We can't own firearms for self defence. It's not a genuine reason. Professionals who need firearms, eg Police, correctives, cash in transit guards etc, can carry firearms for that purpose, but usually only whilst on duty. Most cops leave their firearm at work here. If they do take it home they have to comply with the same storage requirements as everyone else. Firearms are required to be secured in a safe unloaded with the ammunition in a separate, locked container. Violent home invasions in Australia are almost universally drug rips. If you do own firearms, there is no absolute prohibition on using them for self defence, however, if you do use them in self defence, standard Australian self defence law applies. You can even lawfully use an unlawfully owned firearm in self defence. That said there are penalties for the possession of illegal firearms. If you can't justify shooting the person at law, you may be looking at a murder/manslaughter charge. Our self defence laws are robust here, but you just can't kill people for no other reason than they are in your home. The response has to be reasonable and proportionate.
Some of you might be surprised to hear that Firearms are regulated at a state level and the laws are not federal, with the exception of border controls on imports, and that the National Firearms Agreement of 1996 was sponsored by John Howard, our conservative Prime Minister at the time. It followed the 1996 Port Arthur Massacre. Since then Australia has had only two mass shootings, and in both cases it was a domestic violence incident where a licensed shooter killed his whole family before turning the gun on himself.