Green’s Dictionary of Slang

(Aus.) an unexpected piece of good luck.

1881		Bulletin (Sydney) 2 Apr. 9/3: That rhyming fiend would gloat over the ‘throw-in’ as they call it of such an easy task.
1885		Bulletin (Sydney) 17 Oct. 12/4: It would have been a great ‘throw-in’ for Col. Valentine Baker and Lord St. Leonards if they raised magistrates like this in England.
1900		Bulletin (Sydney) 22 Sept. 21/1: The ‘Numurkah (Vic.) Tragedy’ is rather a throw-in for Parson Gladstone. [...] [O]ne young man got fatally injured in falling through a window-pane. Which clearly proves, from Rev. G.’s point of view, that dancing is a devilishly dangerous amusement.
1911		E.J. Brady Tom Pagdin Pirate 53: ‘[I]t [i.e. a murder] ’ud be a throw-in for us [...] I reckon there'd be a big reward and we'd git the money’.