Green’s Dictionary of Slang

1. (Aus.) an informer.

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.
1944		L. Glassop We Were the Rats 133: I haven’t forgot what he did ter me back at Bathurst; how he pooled me with the Q.M. Just a top-off merchant.
1949		R. Park Poor Man’s Orange 190: Pulled up as witnesses by the police, they preferred a fine or jail rather than have their heads kicked in as top-offs.
1959		D. Niland Gold in the Streets (1966) 222: That’s me great, sweet father that would play Judas for the price of his son. The grudge-artist and the top-off!
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xli 4/4: top off: A phizzgig, an informer.
1972		A. Chipper Aussie Swearers Guide 43: Top-off Merchant. An informer.
1977		(con. 1941) R. Beilby Gunner 304: The drinkers stood their ground, jeering. ‘Top-off!’.

2. (Aus. prison) an act of masturbation.

1990

Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Top off. 1. Masturbation.