Green’s Dictionary of Slang

(N.Z.) a very tough, recalcitrant person, a ‘hard case’.

1959		G. Slatter Gun in My Hand 91: He’s a hard shot. Yeah, he’s a woopkacker all right.
1982		M. Shadbolt Once on Chunuk Bair (1990) 66: You wopcacker, Scruf. Scruffy: I told you I could [DNZE].
1988		McGill Dict. of Kiwi Sl. 124/2: woopknacker a hard case, variant spelling of ‘whopcacker’ and once more common, from c.1920.
1993		O. Marshall ‘The Bank Section’ in Ace of Diamonds Gang 122: Yea, well remember the size of some of them. Bloody wopcackers.
2003		McGill Reed Dict. of N.Z. Sl. 233: woopknacker A rough diamond character. From 1920s.