Green’s Dictionary of Slang

1. (Aus.) one who is addicted to cheap wine, an alcoholic.

1946		Canebrra Times 29 Oct. 2/4: His 18-year-old mate, Russell Drinkworth, has the nickname ‘Plonko’.
1949		Mercury (Hobart, Tas.) 16 May 12/3: Drunkenes in Hobart at present is almost confined to the ‘plonkos’ and ‘methos’.
1955		Mirror (Perth) 17Dec. 4/3: Using disguises from a well-dressed woman [...] to a begging ‘plonko’ in the slums of Redfern.
1959		D. Hewett Bobbin Up (1961) 221: Old Plonko Charlie [...] went behind the bobbin boxes, where he kept his bottle of bombo.
1965		W. Dick Bunch of Ratbags 69: We could go and see if there’s any plonkos under Martin’s Bridge and chuck rocks at ’em. [Ibid.] 73: I wonder why he’s got a comb; plonks never comb their hair.
1972		A. Chipper Aussie Swearers Guide 67: Wine fanciers of all descriptions leave themselves open to being called plonk fiends or plonkos.
1980		(ref. to 1920s–30s) Hepworth & Hindle Boozing out in Melbourne Pubs 15: Those who followed the Bacchic way were variously known as plonk fiends or artists, plonkos, winos, bombo bashers, winedots and wyandottes.
2000		Guardian G2 1 June 2: A precocious premonition that his destiny was to be a plonkie.

2. (Aus.) cheap wine.

1955		Argus (Melbourne) 10 Dec. 1/4: Are they out on the plonko again?
1966		D.P. More Golden Road to Cape Reinga 91: Members of the same race [...] who produce every possible kind of drink from Plonko Fino to passable Burgundy.
2001		(con. 1945–6) P. Doyle Devil’s Jump (2008) 201: I drove straight over [...] stopping on the way for a bottle of plonko.