Green’s Dictionary of Slang

1. used in West Australia to refer to the eastern states.

1846		signature of letter in Bell’s Life in Sydney 5 Sept. 3/1: Yours truly, T’Other Side.
1856		Age (Melbourne) 13 Dec. 5/2: An old lag from the otherside, where he had done a long sentence from home, he graduated regularly to Victoria, via Sydney.
1865		C.R. Thatcher Colonial Songster (rev. edn) 77: He stuck up to a gal named Moggy, A big stout lass from t’other side.
1889		Bulletin (Sydney) 5 Oct. 8/2: In the rouse-abouts’ hut […] they always spoke of the Cabbage Garden as ‘Port Phillip’, of the Holy Land as ’tother side.
1904		Sun. Times (Perth) 10 Jan. 8/2: [heading] Beyond the Bight. T’otherside Topics.
1910		Sun. Times (Perth) 19 June 3rd sect. 17/7: As a rule when ’totherside shows open in Perth, the company hasn't got over its seasickness.

2. (Aus.) Tasmania as seen from mainland Aus, esp. Victoria.

1855		Melbourne Punch 2 Aug. 181/1: [The bullock driver] describes the geographical divisions of Australia geometrically, by a reference to its ‘sides.’ Thus he will speak of [...] the other ‘side,’ by which vague expression he is understood to refer to Tasmania.
1882		Border Watch (Mount Gambier, SA) 30 Jan. 5/1: Their ‘helps’ were then generally from ‘’tother side,’ as they called Van Diemen’s Land, and were not very scrupulous about the means they used to keep down the darkies, and it was said that poisoned damper was sometimes resorted to.

3. used in Tasmania to refer to the Australian mainland.

1859		Age (Melbourne) 14 Nov. 5/4: A witness named Yates was amusingly innocent of the existence of an island called Tasmania, but knew it under the less respectable name of Van Diemen’s Land. He had lived at ‘t’other side’.
1900		Leader (Melbourne) 28 July 28/4: ‘’Tother side is Victoria. Don’t you know that much, thickhead?’ .
1945		Baker Aus. Lang.