Australia post couriers cerainly do have police checks,It is the 1st thing done when you apply for the job.

And 99% of couriers do a great job,and are honest people,So do not let the other 1% make you think they are all bad.

When it comes to tracking ,all it takes is a damaged barcode that does not work,which will make your parcel untrackable,and there could be other reasons, as well,But you certainly cannot put lost parcels down to theft without any kind of proof.