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Coins, bank notes and bullion
In addition to this list, we won't deliver bullion, bank notes or coins in any international services. ...
This prohibition applies even if the destination country does accept valuables at their end.
Sending valuable items
Sending a valuable item? Ensure these are safeguarded and okay to send
auspost.com.au
I found that curious. Regulations here in the USA for shipping internationally allow shipping bullion as long as you fill out all the necessary forms AFAIK.