A couple of things to consider. Have you bought anything recently from overseas (esp UK or Europe) where a signature was required? AP can track these so-called tracking numbers but the customer can't once it has left the seller's country. If you phone AP customer service they may be able to tell you more if it's that kind of tracking number.
If the tracking number can't be found on AP or eBay, did you raise a query with Aust Post about an eBay purchase (late, missing delivery, tracking shows parcel not moving etc)? Sometimes these AP numbers aren't tracking at all but customer enquiry lodged numbers for which they want you to rate the response to your query.
