how to show all cities of a country when selected in dropdownlist

I have two dropdownlist combo'x one for country and one for city , now yet i don't have any data in any of them , but i want to show atleast Australia , and Newzeland in the country combo and when Australia is selected its all cities regardless of territories must be shown in City Combo , same for Newzeland ... that's for now only, in future i will be needing or adding more countries too...