Green’s Dictionary of Slang

(Aus.) a mythical figure whose supposed wealth is ready and waiting to bail one out of any problems; also attrib.

1902		H. Fletcher Waybacks 6: ‘Ain’t yer got an uncle in Fiji?’ demanded Dads, with scorn. ‘Ain’t yer got two hundred quid to give to the honest man who will trust you to the same amount?’ [GAW4].
1911		M.P. Adams [bk title] The Rich Uncle from Fiji and Some of his Relatives [...] [note] Ironical exposures of tricks of ‘confidence’ men with fictitious presentation.
1914		H. Lawson ‘Mitchell on the “Situation”’ in Roderick (1972) 716: They were both spielers [...] Their game was anything weaker or stupider than themselves that had cash or property, and when they were in Sydney their uncles lived in Fiji.
1919		Aus. Parlty Debates 90 1919: I assure honorable senators that there is no ‘Fiji uncle’ business about this transaction.
1920		R. Bedford Aladdin and the Boss Cockie 133: Oh! life ’s a stringer-on—the original gold-brick man and bunco-steerer—the only reliable uncle from Fiji— that's life.
1928		A. Wright A Good Recovery 9: I’m beginning to think that rich uncle is like the one from Fiji, eh, Lance? [GAW4].
1943		Aus. Parlty Debates 180 2442/1: [I wonder] whether it resembles a gift from ‘a rich uncle in Fiji’ or a ‘gold brick’ from some other place.