Green’s Dictionary of Slang

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

1908		Sun. Times (Perth) 14 June 1/1: This notorious old shelfer is a kidder of porcine principles.
1938		Press (Canterbury) 2 Apr. 18: A ‘shelfer’ is an informer.
1941		Baker Popular Dict. Aus. Sl.
1988		McGill Dict. of Kiwi Sl. 99/2: shelfer police informer.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].