Green’s Dictionary of Slang

(Aus. prison) a prison chaplain.

[1917		R. Lord Boyd’s Battery 24: Charley [...] the Chaplain [HDAS]].
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Charlie Chaplains. Prison ministers of religion. Has actually been adopted by the ministers at one prison (Long Bay) and is used as a sign on their offices there.