Hunting with firearms is not illegal in Australia. They are relatively common in rural areas, they have their use. All you need is someone stealing guns one way or another and then smuggling them to the city. It’s not like the police will stop you to rummage through your belongings… unless you drive like an absolute reckless imbecile.

I don’t think you can prevent this from happening again. You can definitely take measures to make it more difficult to happen again, ie. police cordoning an area where there is a religious gathering. But there is always a way. You can’t control everything. When it’s not guns it’s a car driving over people or explosives or poisoning or whatever.