Green’s Dictionary of Slang

shelfer n.2

[ext. of shelf v.]

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

[Aus]Sun. Times (Perth) 14 June 1/1: This notorious old shelfer is a kidder of porcine principles.
[Aus]Baker Popular Dict. Aus. Sl.
[NZ]McGill Dict. of Kiwi Sl. 99/2: shelfer police informer.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].