Green’s Dictionary of Slang

(Aus. prison) a veteran prisoner, serving a long sentence.

1856		Age (Melbourne) 13 Dec. 5/2: [A]n old hand, pretending eagerness to escape, will incite a lot of others, - frequently new and young men to join him.
1882		Sydney Sl. Dict. (2 edn) 6: Old Hand, or Lag - An old convict (one who was not a passenger, but who ‘come’).
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 129/1: old hand n. = old lagger.