Green’s Dictionary of Slang

(Aus./N.Z.) a police informer.

1922		Ballarat Star (VIc.) 23 Feb. 2/2: He is known as the ‘shelf’ among the prisoners. He carries tales about the actions of the prisoners to the warders and adds to them a bits on his own account. He is known as a ‘bad dog’ to everyone.
1924		C.J. Dennis ‘The Crusaders’ in Chisholm (1951) 81: ‘’Ere! Wot’s the game?’ ’e yelps. ‘Are you a shelf?’.
1932		Sun. Mail (Brisbane) 13 Nov. 20/8: On the lofty heights to which every ambitious criminal hopes one day to attain, stands the ‘gunman,’ King of the underworld, and at the other the ‘Top-off’ or ‘Shelf,’ the unofficial police pimp, held in contempt by all .
1940		E. Curry Hysterical Hist. of Aus. 175: Actually, my dear pupils, he was a shelf, a fizgig, a top-off, or, to use more polite language, what is known as a police pimp.
1953		Baker Aus. Speaks v. 124: A stool pigeon or informer, otherwise known as a fizgig, fizzer, shelf and topoff.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxix 4/5: shelf: An informer.
1982		G. Newbold Big Huey 75: In that same year a shelf at Mount Eden got cut up for a similar offence [i.e. informing].
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 164/1: shelf (also shelfer) n. an informer, a nark.