Green’s Dictionary of Slang

1. an Australian or New Zealand soldier.

1932		(con. WWI) L. Mann Flesh in Armour 14: [T]here were always diggers [...] passing by with ‘Good day, dig’.
1941		N.Y. Herald Trib. 29 June 9/2: During the last war, the soldiers are known as ‘diggers.’ So revered has this title become that today it is a traditional salute. As the American might say, ‘Hiya, fella,’ the Australian says: ‘G’day, dig’.
1943		Baker ‘Influence of Amer. Sl. on Aus.’ in AS XVIII:4 255: Here are a few of the items included: [...] dig, digger, an Australian soldier.
1975		D. Davin Breathing Spaces 95: ‘Talks just like a dinkum dig,’ Ted said.

2. a general form of address.

1919		Examiner (Launceston, Tas.) 31 July 3/7: It’s grand to hear the ‘Good day, Dig!’ The ‘How’s things, mate?’ [...] in Brisbane-street.
1921		(con. WWI) E. Lynch Somme Mud 72: We have forty fine Australian lads in the mud [...] So long, Digs, So long!
1922		Sun. Times (Perth) 15 Jan. 3/8: ‘Suppose you could keep one down?’ I said. ‘Too right, dig. We’ve been doing a perish all day’.
1931		Mail (Adelaide) 30 May 9/5: ‘Dig’ [is] now used as a vocatrive for either a prsonal friend or stranger.
1944		L. Glassop We Were the Rats 70: ‘Dig,’ he said ominously, ‘tonight is going to be one of those nights you’ll never forget.’.
1947		D. Davin For the Rest of Our Lives 50: How are you, dig?
1962		P. White Season at Sarsaparilla in Four Plays (1965) 110: Digger! Old Dig! Thought you’d surprise me, eh?
1964		B. Hesling Dinkumization or Depommification 134: Good on yer, dig!
2003		McGill Reed Dict. of N.Z. Sl. 61: dig/digger/old dig Friendly greeting, usually male to male. Specifically an Anzac soldier or returned Anzac serviceman, from WWI the Australians preferring ‘digger’, New Zealanders ‘dig’.