Green’s Dictionary of Slang

(Aus.) water.

1863		Melbourne Punch 4 June 2/1: Dr. Macadam astonished Fitzroy with the Monsters of the Deep shown in a drop of the Yan Yean, under the Oxy-Hydrogen Gas Microscope.
1877		Bendigo Advertiser (Vic.) 20 Jan. 1/1: The Yan Yean is laid on but is intermittent in supply.
1889		Queenscliff Sentinel (Vic.) 3 Aug. 3/1: I gave him a nice new, comfortable drop of Yan Yean to sleep in every night.
1904		Wkly Times (Melbourne) 7 May 21/5: [of the melbourne mains supply] He may still take his socialistic beer - the opulent fatmen, of course, providing the money, as usual - and ascribe his ‘hiccupping’ to water. If he say Yan Yean, he may be believed.
1909		Punch (Melbourne) 11 Feb. 35/3: On Wednesday evening, 17th February, the Hawthorn Club hold their annual carnival by the aid of the fitful gleams from the incandescent lights and a supply of Yanyean.
1916		‘Turner O. Lingo’ Aus. Comic Dict. 5: Yan Yean - The author of the Melbourne Water Rate, and mother of that false rascal Water Meter.
1932		Sun. Times (Perth) 9 Oct. 4/7: The Yan Yean reservoir is the source of Melbourne’s water supply, and instead of asking the barmaid for water with which to break down his whisky, he asked the Hebe for a drop of Yan Yean.