The Austral Realm, which is made up of Australia and New Zealand, has unique flora and fauna because it spent millions of years in which it was not connected to the rest of the world in any real way. This allowed unique flora and fauna to evolve and it prevented them from being displaced by plants and animals coming in from the rest of the world.
For a long time, Australia was part of a supercontinent known as Gondwanaland. But then that supercontinent began to be pulled apart by plate tectonics. When this happened, most of the rest of the world stayed connected to some degree but the Austral Realm did not. For about 40 million years, the Austral Realm was almost completely separated from the rest of the world. The only plants or animals that could get to the Austral Realm were those that could fly long distances or could float on the oceans.
This meant two things. First, it meant that species could evolve differently in the Austral Realm. Second, it meant that species from outside the realm could not come and displace native species. For example, in the rest of the world, mammals outcompeted marsupials and led to their extinction. In the Austral Realm, this did not happen and so marsupials continued to prosper there.
Thus, the Austral Realm has unique flora and fauna because it was separated from the rest of the world for millions of years.