Green’s Dictionary of Slang

topper(-off) n.

[top (off) v.]

(Aus./N.Z. prison) an informer.

[Aus]K. Tennant Joyful Condemned 223: Her pet magpie was detested by the crims as ‘a bigger topper-off than the screws.’ They vowed it ran squawking to inform on anyone.
[NZ]D. Looser ‘Boob Jargon’ in NZEJ 13 36: topper n. Nark.