Green’s Dictionary of Slang

(Aus./US) an informer.

1944		Maurer & Baker ‘“Aus.” Rhyming Argot’ in AS XIX:3.
1960		J. Franklyn Dict. of Rhy. Sl. 46/1: cabbage hat Rat (an informer).
1972		G.W. Turner Eng. Lang. in Aus. and N.Z. 101: Cabbage hat for an informer or ‘rat’.
2002		Pete’s Aussie Sl. Home Page 🌐 cabbage tree hat: a rat, an informer.