To add a new currency to the business: from the Accounts menu select Multiple Businesses, then click Add. The Startup Wizard will run. It is recommended you name the new business currency something logical like YourCompany - Australian Dollar. Remember, treat it as an entirely new business.
Assign appropriate tax rates and currency notation in the Options settings on the Tax and Regional tabs. See the Options ~ Tax and Options ~ Regional help topics in this manual for more details.