Green’s Dictionary of Slang

1. (Aus.) to fall, thus to fail.

1915

Honk! 4 Jan. 4/2: Have you heard about the paper / I presume its newly born [...] And its coming very shortly, / But I hope it don’t go plonk, / And its going to be a bonza.

2. to sound, thus to signal a start.

1925

N.Z. Truth 11 July 13: As soon as the whistle goes plonk, off it goes .