Amazon’s terms and conditions clearly state that you responsible for all Custom Duties
“You are responsible for assuring that the product can be lawfully imported to the destination country. When ordering from Amazon.com, the recipient is the importer of record and must comply with all laws and regulations of the destination country. Orders that are shipped to countries outside of the United States may be subject to import taxes, customs duties and fees levied by the destination country. The recipient of an international shipment may be subject to such import taxes, customs duties and fees, which are levied once a shipment reaches your country. Additional charges for customs clearance must be borne by the recipient; we have no control over these charges and cannot predict what they may be. Customs policies vary widely from country to country; you should contact your local customs office for further information. When customs clearance procedures are required, it can cause delays beyond our original delivery estimates.”
and that if you select Priority Shipping they will actually include any fees if required during the checkout
“All products shipped to these countries via Priority International Shipping, and in certain cases product shipped via Standard or Expedited International Shipping, are included in the Amazon Global Program and an import fees deposit is collected on them.”
So How Much GST Do I have to Pay?
Australian customs do not charge GST on personal goods valued under $1000. GST and other duties are payable on Alcohol and Tobacco products but none of these items are sold by Amazon.
You can read more about the GST rules as they apply to importing goods into Australia.