Green’s Dictionary of Slang

(later use N.Z.) an inadvertent display of one’s underclothes, orig. those made of red or brown flannel.

1937–84		Partridge DSUE (1984) 911/2: late C.19–early 20.
1968		G. Slatter Pagan Game (1969) 51: The poppy shows with the Robertson tart in the pine plantation.
1973		Aus. Women’s Wkly 1 Aug. 13/2: Any intimate glimpse of feminine garments [...] was called a ‘poppy show’.