Question

why is australia called last frontier

Answer 1

Antarctica  is the last frontier on the planet
Australia is not called the last frontier.
The last frontier , Australia is actually a book. So you might have heard that

Answer 2

Australia is the smallest continent, largest country and biggest island on the earth. It is situated between the Indian Ocean and the Pacific Ocean.

Sometimes, it is also called as 'Last Frontier', probably because it was the last continent that was discovered by Europeans.

However, the continent is not the last to form, becuase the rocks found in the foundation of the island are as old as the Paleozoic and Precambrian age.