Green’s Dictionary of Slang

horse and donk n.

[rhy. sl. = plonk n.2 (1)]

(Aus.) cheap wine.

[Aus] ‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/2: horse and donk: Wine. Rhyming slang for plonk, a cheap wine.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 31: Horse and Donk (Wine) Plonk.