(Thanks to Nanette Tobin and Deb Dempsey.) Cheap wine; more generally, wine or alcohol of any kind. The Oxford English Dictionary cites Australian use beginning the 1930s, with the first U.K appearance (in inverted commas) in 1967. The OED speculates that the etymology may be “a colloquial or humorous pronunciation of blanc.”
Department of Plonk (Heading to a New Yorker Talk of the Town piece about a Manhattan winery, September 26, 2005)/[The Australian wine industry] did, after all, in the 1970s and ’80s, transform a moribund, 100-year-old industry making widely mocked plonk—Monty Python fans will recall “the prize-winning Nuit San Wogga Wogga, which has a bouquet like an armpit”—into the epitome of a modern, high-production industry. Businessweek.com, March 24, 2011.
notoneoffbritishisms.com 
If the first UK usage was in 1976, was it Rumpole? If so, it would be an apt moment to celebrate his sardonic description of the wine as Chateau Thames Embankment.
Eric, I made a mistake in the original post (which I have now corrected): the year was 1967, not 1976, and it was this line in the Daily Telegraph: “Surely the word ‘plonk’ is onomatopoeic, being the noise made when a cork is withdrawn from the bottle?”
The reliable Online Etymology Dictionary dates plonk to 1874 (verb form) and 1903 (noun), and describes it as “imitative”.
Love this blog, love Rumpole, and I use the word plonk often and have done for yonks (another Britishism you might add) beginning when I described the wine I drank in my misspent salad days.
To Plonk: To place something somewhere – “just plonk it down anywhere”. It also means cheap alcohol, usually wine
You may in passing like to know that in come circles, “Plonk” is use as a derogatory term for a junior or new team member. In earlier decades, it would’ve been used to refer to a female police officer by her male “seniors” and colleagues. It stands, in this context, for Person of Little or No Knowledge.
French brandy bottles have the initials “V.S.O.P” stamped on them, the commonly belief that this refers to “Very Special Old Plonk” is entirely unfounded…
Originaily, I think it was almost certainly an alternate (& humorous) spelling/ pronunciation of French “blanc” , ie a cheap white . Later, it came to mean any cheaper wine, white or red.