Green’s Dictionary of Slang

mainland n.

(N.Z.) the South Island.

[NZ]Journeys 34 56: I’m sure South Islanders are right in claiming that they live on the ‘mainland’ [DNZE].
[NZ]R.W. Winks These New Zealanders 54: One last warning: We Americans promise not to confuse the South Island with The Glorious Mainland.
[NZ]W.J. Cameron N.Z. 33: The South Island is sometimes called Te Waka o Maui (the Canoe of Maui) which suits South Islanders’ sense of pride in being on the ‘mainland’ [DNZE].
[NZ]P. Wilson N.Z. Jack 106: Well, so Otago won [...] A victory for the good old Mainland.
[NZ]McGill Dict. of Kiwi Sl. 71/2: Mainland, The the South Island, self-declared.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

In derivatives

mainlander (n.)

(Aus.) one who lives on the mainland of Australia (rather than Tasmania).

[Aus]Advocate (Burnie. Tas.) 4 Jan. 7/7: STRAIT SERVICE. Mainlanders’ Disgust. Company Criticised. Two Melbourne visitors [...] who were in Devonport yesterday, were free in their expressions of disgust at the Strait shipping service [etc].
[Aus]Baker Popular Dict. Aus. Sl. 45: Mainlander, a person living on the Aust. continent, in contrast to one living in Tasmania or an island dependency.
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 235/2: mainlander – a person who lives in Australia proper. Used generally by the Tasmanians to refer to people who live on the continent.
[Aus]P. Adam-Smith Barcoo Salute 32: The people of the island are Tasmanian [...] tough and tenacious as the ‘mainlanders,’ as they call people from ‘over the other side’.