Green’s Dictionary of Slang

(N.Z. prison) an ally, e.g. in a fight.

1893		Illus. Sydney News 11 Mar. 17: She, a light, quick child of 5, had been invaluabl to him as a backstop.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 11/1: backstop n. one who looks out for another, e.g. during a fight or a crimina job.