Australia

Australia is an independent nation comprising of the mainland continent Australia and the island of Tasmania. It may also include many of the same external territories as it owns in the real world.

History
Little is known of Australia's history in Code Geass, as it plays no full part in the storyline. In several world maps from the series, Australia is shown as the only region not controlled by one of the three main powers, after the Middle Eastern Federation was defeated by the Holy Britannian Empire in Stage 5. Even after the formation of the United Federation of Nations after the dissolution of the Chinese Federation and the collapse of the Europia United, Australia is shown to still retain its neutrality and hasn't been conquered by either side.

What do we know?
All we can say for certain is that Australia is an independent country, and that it is possible to migrate to Australia by plane.

What can we assume?

 * Australia has to have been discovered to be shown on world maps.
 * It will need to have been properly inhabited for it to remain neutral. Primitive Aboriginal weapons alone would be no match for modern technology. Australia needs to have a strong population, with its own army and modern weapons, to be able to remain neutral. We can assume therefore that either Aboriginal society is far more advanced in this timeline or another set of colonists set up shop in Australia and have managed to ward off other factions with success.
 * If it is possible to take a plane to Australia, then we have to assume that it has airports. We can also assume that Australia maintains diplomatic relations with both the Chinese Federation and Britannia, which would help in its neutrality.

Mentions

 * Stage 15 sees Mao shoot C.C. several times and state that he has built a big house in Australia and intends to take C.C. with him. To get to Australia, they must take a plane, and Mao explains that she is too big to take with him, so plans to make her 'compact' by cutting her up with a chainsaw.
 * In Turn 23, Schneizel el Britannia reveals his plan to bring peace through the threat of annihilation on every warmongering nation. Yellow dots on a world map identify targets. One such dot appears in the approximate location of Sydney, Australia's largest city in the real world.