I remember posting somewhere else on this site, that replica weapons (like Megatron gun toys) may not require a permit to own, but it is illegal carrying it in public.
(and the permit for ownership can sometimes be up to interpretation, depending on who you call up to ask)
Plus, it is still worth considering that most of the international mail comes through Sydney to be processed by Customs, so while it is currently in NSW, they will treat it as illegal the moment it gets into that jurisdiction. But if you write to them if they notify you of the seizure, they should release it to you.