Fellow Australian here, and I've always viewed our biosecurity as being pretty tight and effective, too.
Apparently however small innocuous packets/letters don't receive quite as much screening, as I've heard of people getting marijuana seeds in the mail just fine.
Considering how one or two seeds can be enough if the purpose is simply to acquire the species it seems like an impossible task to prevent that.
It's far more difficult than drug smuggling which only works in some reasonable quantity - and becomes more like information smuggling in biological form where you only need one copy.
Apparently however small innocuous packets/letters don't receive quite as much screening, as I've heard of people getting marijuana seeds in the mail just fine.