PO staff are not experts for eBay listing or calculators.  They put parcels on their scales, cube it, enter postcode and the price comes up for them.  To check individual parcels for postage costs you would be better off looking at AP site: http://auspost.com.au/products-and-services/postage-assessment-calculator.html

As said above, if you look at your listing you only see postage to your postcode.  However, if you log out, look at your listed item, and click on the tab for postage then you will be able to postcode - any postcode and see what comes up.  Is that what you meant in your original post?  I never look at anything while logged out; forgot you can do that.  🙂