What if I built one out of paper well enough to look like the real thing and carried it down the street in some Australian city? Be thrown in jail for brandishing a fake firearm?
If it look real enough that a reasonable person would believe it to be a firearm, then it would depend on whether or not you have the right licence to own it, and were carrying it appropriately. If you have the right type of licence, provided you are not carrying a firearm in a dangerous manner, you're pretty much fine. People are allowed to own firearms and they are allowed to move them from place to place. Sometimes that involves carrying them in public.
Of course, it you are carrying it in a dangerous manner, you're going to be lucky to reach jail.