Green’s Dictionary of Slang

cabbage-tree hat n.

also cabbage hat
[rhy. sl. = rat n.1 (1c)]

(Aus./US) an informer.

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