After sending some Gold Bullion using Australia Post, I discovered from reading the small print on the back of the Registered Mail document that Australia Post do not insure Coins, Gold or Silver Bullion.
I went back to my local Post Office and asked why they had accepted insurance on my article as I clearly stated that I was sending 'Gold Bullion'. They told me that they did not know about this exclusion clause.
I had sent my parcel Registered Mail and Insured. If the parcel had been lost then I would not have been able to make a claim.
If you still want to send Coins or Bullion using a Postal Service as this is far cheaper than a Courier Service, mark your parcel as 'Industrial Metal'. You can then insure it for it's full value. Do not mention anywhere that it is Coins or Bullion.
Alternatively you can send it using the more expensive Courier Services.
Hope this article might help you.