Green’s Dictionary of Slang

1. (also zack-bit) a sixpence, a very small sum of money; A5$.

1898		Bulletin (Sydney) 1 Oct. 14: 6d., a ‘zack.’.
1901		Sydney Sportsman (Surry Hills, NSW) 6 Feb. 1/1: [They] find themselves at a cafe de zack or tray.
1912		Bulletin (Sydney) 22 Aug. 9/4: One is expected to tender a bob, or at the very least a ‘zack,’ to the person who hinders you from getting into your overcoat at a Melbourne theatre.
1913		Truth (Sydney) 16 Feb. 7/8: Crawlin’ for a stray zacstiver, / Or a miserable thrum.
1913		Sport (Adelaide) 22 Mar. 12/2: They Say [...] That Tommy H, the sport, murdered a zac [...] Don’t be extravagant, Tommy!
1917		Rising Sun 5 Feb. 3/1: Y is yearning for Australia so fair; / Z’s the zack-bits for the drinks when we’re there.
1921		Barrier Miner (Broken Hills, NSW) 2 Aug. 12/4: Sharing his meals with the ‘mung’ at his heels / And with never a care or zack.
1924		C.J. Dennis ‘Nocturne’ in Rose of Spadgers 55: I knoo the sort of gold – / Priced in the brumy shops four an’ a zac.
1931		‘William Hatfield’ Sheepmates 72: There’s too big of a mob fer one man to shout the house on his pat at a zac a pop, so you shove in a deaner a nob and flip the rats an’ mice, see?
1941		K. Tennant Battlers 171: That’s three sons he’s got working [...] and I’ll bet me last zac most of the others are uncles or nephews.
1949		L. Glassop Lucky Palmer 68: I’ve got a zack [...] We’re a couple of no-hopers all right.
1953		D. Cusack Caddie 225: I thought of them forking out their hard-earned treys and zacks and bobs and nearly cried.
1957		‘Nino Culotta’ They’re a Weird Mob (1958) 147: Got any Minties? [...] Give us a zac’s worth.
1959		D. Niland Big Smoke 183: How’s it for a feed, Gus? I on’y got a zack.
1961		R.S. Close With Hooves of Brass 78: ‘Drop to a zac a point and I’ll be in it too,’ Ziff said, getting up.
1966		P. Pinney Restless Men 71: If crabs was a zack each you’d be a millionaire.
1973		(con. 1930s) F. Huelin ‘Keep Moving’ 2: There’s some useful buskers in Adelaide [...] I tried it in Hindley Street — didn’t get a zac.
1983		(con. 1940s–60s) ‘Fanny Bay’ in Hogbotel & ffuckes More Snatches and Lays 50: But what will cost you twenty quid in Lower Crown Street, / Will cost you half a zac in Fanny Bay.
1984		B. Ellem Doing Time 200: zack: a six-months sentence.
1988		McGill Dict. of Kiwi Sl. 128/2: zack a sixpenny coin c.1890, possibly corruption of ‘six’.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers xx: The Flea was about to invest every zack he owned.
1999		R.G. Barrett Wind & Monkey (2013) [ebook] ‘I refuse to give the sour-faced, fat prick another zac’.
2013		T. Peacock More You Bet 6: An army of thrifty housewives [...] could have a ‘zack’ (that is, sixpence), or a ‘deaner’ or a ‘bob’ (that is, one shilling) ‘each-way’.

2. a six-month prison sentence.

1911		Truth (Brisbane) 22 Jan. 10/4: To the prisoner [...] a sentence of one month is a ‘moon’; of three months a ‘drag’; of six months, a ‘sixer’ or ‘zack’; twelve months a ‘stretch’; and five years a ‘fin.’.
1941		Baker Popular Dict. Aus. Sl.
1955		N. Pulliam I Travelled a Lonely Land (1957) 242/2: zack (zeck) – a sixpence; also six months in jail.
1983		Aus. Journal of Cultural Studies May 91: One year: A Sleeper. / Six months: A Zac. / [...] Indefinite detention at the governor’s pleasure: The Key.
1990		see sense 1.