Yes, that's the issue, which is why I've now noted about authorising someone else who can pick it up, and post it onto you.
And if this had been released a few years ago and was caught, I'd most likely would have had it captured at Sydney instead of Brisbane, as most or all Brisbane-bound mail was coming through there until recently.
They don't scan everything... but if they randomly scan your item and capture it, now you know what to do to get it released.