Copper handles a business dealing with multiple currencies as a different business for each currency. For example you could create one "business" called YourCompany - USD and another YourCompany - Euro. Each could handle sales tax, VAT separately and you can print reports entirely separately for the US dollar account or the EURO account. Each account must be set up separately with its own salesperson, items etc.

To add a new currency to the business: from the menu, select Copper -> Multiple Businesses. Then click Add. After you click Add the Startup Wizard will run. Then you can choose different currency from the Options -> Regional under the Regional Format Settings section. It is recommended you name the new business currency something logical like YourCompany - AussieD. Remember to treat it as an entirely new business.