Green’s Dictionary of Slang

(Aus.) an ejaculation of semen; thus have an empty, to masturbate.

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 9: I haven’t had an empty since the cheese stalled over a month ago.
1993		Smith & Noble Neddy (1998) 189: One day he called into a brothel for an empty and, by coincidence, his wife was working there. Yet he still did nothing about it. Weak mongrel.