Which of these countries is often called the "Down Under"?

The term ‘Down Under’ refers to Australia but sometimes the term is also used with reference to New Zealand. The term is often used by outsiders, but not Australians. Officially, Australia is known as the Commonwealth of Australia. The capital of the country is Canberra while the largest city is Sydney. It is a monarchy led by Queen Elizabeth II. The Governor General is Sir Peter Cosgrove. Australia’s population in 2018 is estimated to be about 24 million.

Source: World Atlas