Green’s Dictionary of Slang

(Aus.) a thin man.

1831		Navy at Home I 134: You must prove that, old herring-gut, eh! — and now get on.
1909		Sun. Times (Perth) Supp. 19 Dec. 25/1: Reggie, with his effeminate face and narrow waist (‘Herring gut’ - the boss of the sox department called it), was hard to imagine as [...] a knocker-out.
1919		W.H. Downing Digger Dialects 28: herring-gut — A thin man.