Green’s Dictionary of Slang

(Aus.) underpants.

1972		A. Chipper Aussie Swearers Guide 55: Take Down. A fraud. When an Aussie con man (trickster) does a take-down job on you, you have been caught with your tweeds (trousers) and underchunders (underpants) down.
1992		K. Lette Llama Parlour 83: I glanced at his tight pants and wondered whether or not he was wearing any underchunders.