Green’s Dictionary of Slang

(Aus./N.Z. prison) an act of sexual intercourse.

1974		J. Hibberd Peggy Sue (1984) 13: Bruce: Like a root? Peggy: Eh? Bruce: A Bill Buck [Simes:DLSS].
1981		C. Gordon Night in Arms (1983) 45: Don’t tell old Rat nothin’ about sex, pal. I wrote the fuckin’ Karma Suture! Moxy: Sutra. Rat: Yeah, whatever. The book of billy bucks, mate, whatever that’s called [Simes:DLSS].
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 18/1: billy buck n. an act of sexual intercourse.