Green’s Dictionary of Slang

thud n.

(Aus.) a bad fall (usu. in fig. contexts).

[Aus]Aussie (France) VIII Oct. 7/1: ‘What do you boys mean, exactly, when you refer to a gutzer?’ asked a young lady at the Y. Emma C. Ack, pausing beside us, and leaning against a chair. [...] ‘It means a ‘thud’,’ explained Matthews. / ‘A fall-in,’ added O’Brien, helpfully. / ‘A bad sort of failure,’ I suggested.
[Aus](con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: thud. See Gutzer.

In phrases

come a thud (v.)

(Aus. / N.Z.) to fall from grace; to meet disaster.

[Aus]Sun. Times (Sydney) 31 Aug. 7: [advert] Dig in at once or you will come a thud.
[Aus]Advocate (Burnie, Tas.) 5 June 7/2: He’s going to scale sport to-morrow and pinch off to the flicks. He’ll come a thud.
[Aus]Sydney Morn. Herald 15 Mar. 4/2: ‘That’s where you come a thud,’ said Bill. ‘Cos you see, Miss Clever, I happen to know the name’.
[Aus]Sydney Morn. Herald 9 Oct. 2/4: ‘I expect Mr Hawke to come a thud in this State’.
[NZ] McGill Reed Dict. of N.Z. Sl.