Why Do Australian Customers Need To Pay GST?

As we are an Australian GST registered company we are required to charge 10% GST on all transactions to Australian based customers.
The main question we are asked about this is why we do this even though we are selling in US Dollars.
The GST tax is calculated irrespective of currency, 10% is still 10% whether it is in AUD or USD and after the conversion, it is exactly the same amount.
We hope this clears up why Australian customers need to pay GST even though the transaction is in USD. For more specific questions please consult your tax accountant.