Green’s Dictionary of Slang

(Aus.) a fluke, thus flum job; also as v.

1977		J. Ramsay Cop It Sweet! 36: FLUM: Fluke.
1990		R.G. Barratt ‘Aunty’s Aces’ in What Do You Reckon (1997) [ebook] Naturally, they have to flum a lemon now and again, like Jane Singleton.
1996		J. Byrell Liars, Urgers & Coat-Tuggers 152: And that was a pure flum job since Lennie’s Aunty Vera [...] plonked on the wrong number.