Green’s Dictionary of Slang

(N.Z./UK, mainly juv.) a club, a bludgeon.

c.1938		in DNZE (1998) 220/1: Donger-knocker was a Marlborough nursery word for a stick or other ‘bludgeon’ usu. the implement of a humorous threat (Ed.).
1980		Cowley & Melser Hungry Giant 2: ‘1 want some bread!’ roared the giant. ‘Get me some bread, or I’ll hit you with my bommy-knocker’ [DNZE].
1988		McGill Dict. of Kiwi Sl. 38/1: dommyknocker a stick to beat other kids with, aka bommyknocker; origins unknown.
2003		McGill Reed Dict. of N.Z. Sl. 65: dongyknocker A club, cousin of ‘dommyknocker’, invoking the concept of a dong or knock, such as a hunter might use to dispatch a possum.
2007		R. Milward Apples (2023) 81: [Y]ou hoped Brandon would appear round the corner with a bomby-knocker.