(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.