Green’s Dictionary of Slang

(N.Z.) underpants.

1981		G. Johnston Fish Factory 88: ‘I hope he hasn’t changed, but I suppose he could have.’ [...] ‘Yeah. Under-dungers once a week and socks every fortnight,’ Jamie retorted. [Ibid.] 173: Thought the whole place was going at first. Man nearly filled his under-dungers.
1988		McGill Dict. of Kiwi Sl. 117/1: underdungas underpants, possibly from dungas as short for dungarees.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].