I've found Aust Post to be pretty reliable on the whole.

Just today i received an item sent in a large envelope (no tracking), hand written address with correct street/suburb/postcode but then it listed the wrong state, Qld instead of Vic.

It still got to me within about 3 or 4 days from interstate.

If correctly addressed envelopes are regularly going missing, that sounds suss to me. The occasional one maybe, but never one in ten.