In case of Australia the name country and continent is same.why?

The name "Australia" is used for both the country and the continent because the continent is almost entirely occupied by the country of Australia. When Europeans first arrived in the late 18th century, they named the continent "New Holland". However, in 1817, the British explorer Matthew Flinders suggested the name "Terra Australis", which was later shortened to "Australia". This name was adopted by the British government and became the official name of the country in 1901 when the Commonwealth of Australia was established.

It is quite common for countries and continents to share the same name. Examples of this include Africa, Asia, and Europe.