Green’s Dictionary of Slang

(N.Z. prison) someone is always well-supplied with food which is available for sale (cash or credit) or barter.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 52/2: dairy, the n. an inmate in possession of a steady supply of food (P119 goods) for sale to other inmates on a cash, barter, or credit basis.