Green’s Dictionary of Slang

(N.Z. prison) an informer, i.e. an ‘Automatic Teller Machine’.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 8/2: ATM n. an infonner, a nark. [stands for Automatic Teller Machine, and thus an infonner as an 'automatic teller'. This term is often used in conjunction with the expression: ‘You should work in a bank’].