What's the origin of the phrase: "Down Under"?

[Anchela]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #17 "Down to Earth", question 10

They've had enough of the bad climate and high taxes here and have gone to live down under.

(a) in America
(b) in China
(c) in Australia
(d) in India

English Idioms and Expressions, Advanced Level

ESL/EFL Test #17 "Down to Earth", answer 10

They've had enough of the bad climate and high taxes here and have gone to live in Australia.

Correct answer: (c) in Australia
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What's the origin of this phrase? Why does it stand for Australia?

Thank you in advance.
Anchela

[Conchita]

Australia and New Zealand are sometimes referred to as 'Down Under' because of their geographical location below the equator. They are also humourously called 'the Antipodes'.