Green’s Dictionary of Slang

(Aus.) a heroin addict.

1955		B. Appel Tough Guy [ebook] You’re a stinkin’ drunk You’re a cheesy junk.
1975		Bulletin (Sydney) 26 Apr. 46: Even the popular junky idiom is ignored even though. on the surface, one would think a junk to be a natural knock-about. Not so. Most thieves distrust dope addicts.