Australia is colloquially known as "the Land Down Under" (or just "Down Under"), which derives from the country's position in the Southern Hemisphere, near the antipodes of the United Kingdom. The term was first recorded in print in 1886, and was popularised internationally by the 1980 song of the same name by Men at Work. Other less common nicknames include "Straya" ("Australia" pronounced in an exaggerated Strine manner), and "Aussie", which is usually used as a demonym, but occasionally extended to the country as a whole (especially in New Zealand).

Source: Wikipedia