[jameshart]

Which implies that while the whole of ‘Australia and New Zealand’ can be referred to as ‘the antipodes’, Australia or New Zealand alone should be called ‘an antipus’.

[LegionMammal978]

"Antipous" has been used as a singular form of the word in some 19th-century works [0].

[0] https://en.wiktionary.org/wiki/antipous#English