The mail service, and the customs agent from both country will not open the package without a reason. You will find a similar answer from a search online. It'll go through an x-ray machine. If something looks suspicious, they will then have a reason to inspect the item before it leaves or enters a country.
USPS inspected my items before and they will leave a note indicating that they have done so, and tape the opening with their branded tape. Australia may or may not have the same procedures.
