The Customs Duty rate is determined by the tariff classification that covers the goods.
Without taking into account Free Trade Agreements, most goods attract 5% duty.
This is payable against the FOB (Free on Board) value of the goods.
FOB value is the cost of the goods plus any additional costs incurred in getting the goods ready for export and delivered to the export port.
If you buy goods on a FOB contract this price will be the Customs Value of the goods on Import.
Customs use their own rate of exchange on the day the goods were exported to convert foreign currency into Australian dollars for calculation of the Customs Value.
GST is payable on the CIF (Cost, Insurance and Freight) value plus any Customs duty payable, so this includes all costs in getting the goods to the port in Australia plus any duty payable.