Technically, what is mainland Australia?

At about 3 million square miles (7.7 million square km), Australia is the smallest continent on Earth. In fact, it’s closer in size to Greenland than it is to South America. Although Australia is sometimes called an “island continent,” most geographers consider islands and continents to be separate things. An island is a mass of land that is both “entirely surrounded by water” and also “smaller than a continent.” By that definition, Australia can’t be an island because it’s already a continent.

Source: Britannica