Green’s Dictionary of Slang

(Aus.) a non-union labourer, a strike-breaker, spec. a member of the Permanent and Casual Waterside Workers’ Union.

1947		J. Morrison Sailors belong Ships 29: An air of strained expectancy pervades the great bleak shed. In the outer divisions – ‘Jacks’, ‘Seconds’, ‘Unattached’, ‘Blanks’ – the bell is hardly heard over the babel of four thousand voices [AND].
1955		J. Morrison Black Cargo 26: Procedure at a pick-up is for the foreman to offer the job first to Federation and Jacks in the ratio of 60 percent to 40 per cent, according to the terms of the ’28 strike settlement.
1982		in Lowenstein & Hills Under Hook 88: The Permanent and Casual Waterside Workers’ Union (popularly known as jacks or scabs).