Green’s Dictionary of Slang

(Aus./N.Z.) to inform against.

1931		Eve. News (Rockhampton, Qld) 27 May 3/1: Crooks are ‘topped off’, or ‘shelved’ when. an informer secretly tells the police where the culprits are to be found.
1939		K. Tennant Foveaux 312: There was too much danger that a squarehead would top-off to the police in a jam.
1941		K. Tennant Battlers 13: There’s always some cocky’s son who wants to buy the skins off you at sixpence each and sell them at three bob himself, and if you won’t give them to him, he’s likely to top you off to the police.
1953		I. Hamilton Till Human Voices Wake Us 41: You couldn’t trust him not to top you off.
1982		G. Newbold Big Huey 255: top off (v) Inform against. Australian, since about 1920.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Top off. 2. To betray or inform upon. Thus a topper is an informer.
2003		McGill Reed Dict. of N.Z. Sl. 215: top/top off/top on To inform on somebody, originally Australian police informer.