Green’s Dictionary of Slang

(Aus./N.Z.) anything, usu. mechanical, that is worn out or malfunctioning, e.g. an old car; also as adj.

1988		McGill Dict. of Kiwi Sl. 41/1: dunga, dunger trouble, in phr. in the dunga; perhaps extension of word ‘dung’. – dilapidated object, such as house, car or contraption, from engine going ‘dunga, dunga, dunga’, the word serving to indicate the object is useless.
2001		OnLine Dict. of Playground Sl. 🌐 dunga adj. useless, worthless, e.g. ‘What a Dunga car, a real heap of shit!!’.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].