Green’s Dictionary of Slang

plonk n.1

1. a blow, a punch.

[UK]Manchester Eve. News 27 Sept. 8/2: ‘This man seemed to come from nowhere, and I got one plonk straight on the jaw’.

2. (Aus.) a bet, a wager.

[Aus]J. Byrell (con. 1959) Up the Cross 38: They all had a solid plonk with the same bookies.
[Aus]J. Byrell Lairs, Urgers & Coat-Tuggers xx: Old Horrie had good and just reason to question The Flea’s intended plonk on Unpainted [ibid.] 213: [H]e had the best part of 30 bucks to have a plonk with.

In phrases

just the plonk

(Aus.) satisfactory, as required.

[Aus]Brisbane Courier 29 May 6/3: ‘Just the glassy marble’ [...] ‘Just the glassy alley,’ ‘Just the juicy chop,’ ‘Just the blob,’ ‘Just the shiny shilling,’ ‘Just the plonk’ are only a few of its offshoots.