After the outbreak of the plague, the leaders of Australia threw millions of dollars at their nations' top scientists, hoping against hope that they would be able to develop a cure. The scientists did everything they could, but the plague was unstoppable, wiping out an incredible percentage of the population of humans and our closest primate relatives.
The scientists, now panicking and not thinking straight, crossbred humans with a variety of animals, reasoning that if they could make humans inhuman, the plague- which targeted only apes- would no longer pose any danger. And they were correct; the first subjects were completely immune to the plague. Despite this success, however, the scientists knew that their proposal to dehumanize humanity would be a tough sell, and the Australian government was unlikely to approve of their use of vaccine research funds for such an endeavor.
When the researchers presented their plan to the public, there was a national outcry. Most Australians were upset that the scientists' plan would not save a single infected person (genetic engineering is rather difficult to perform after the subject is already fully developed), and were extremely unwilling to subject their unborn children to such a procedure. The group most willing to go along with the plan were the Aborigines, and as time went on the human descendants of the settlers died off, unable to resist the plague, while the hybrid children of the Aborigines grew strong and thrived.
One tribe, the Gadigal, grew at a much faster rate than the others, and soon the Gadigal were the most powerful & populous group on the Australian continent. The Gadigal Leader, Jerara Tiwart, organized a meeting of all the tribes of Australia, and after nearly a week of fierce negotiations, convinced them to unite under a single nation, with him as the King.
That was many, many years ago, and King Jerara Tiwart has since become a national hero, often compared (in the Gadigal Region at least) to Alexander of Macedon or Emperor Napoleon. A cult sprung up declaring him and his house the incarnation of the almighty on Earth, and the ruling family, direct descendants of his, encouraged the spread of the religion, knowing that the people would be far less likely to question their rule if they were seen as divine. Queen Pierah Tiwart, an environmentalist, used her status as head and God of the state church to forward her political beliefs, and incorporated environmentalist ideology into the Tiwart cult; Queen Pierah was also the first Tiwart monarch to take the title 'Mother;' every ruler of the Gadigal Region since has been known not as King or Queen, but 'Father' or 'Mother.'
As the Gadigal Region grew in power, the began expanding outwards, shunning the unforgiving deserts of Outback in favor of New Zealand and New Guinea, both islands now virgin territory after having been almost totally depopulated by the plague. They also established naval outposts throughout Indonesia, where explorers heading north brought back tales of strange lands rules by Nationalists & CEOs; thus, the Gadigal learned that theirs' was not the only nation to emerge from the plague intact.
As countries the world over begin to stabilize, the Gadigal are prepared to take their place among the great powers of the world. Being an island nation has advantages, and so long as the Gadigal Navy reigns supreme, the Gadigal Region is sure to remain a beacon of stability & prosperity in this new Multipolar World.