Green’s Dictionary of Slang

(Aus. campus) an especially foul-smelling piece of excrement, usu. produced following heavy drinking.

1998		Woroni (Canberra) 1 Mar. 26/2: Are you going to be getting nasty surprises when that After Grog Bog refuses to make it pass the U-Bend?
2004		J. Lambert Macquarie Aus. Sl. Dict. 1/2: after grog bog a particularly noisome excrement produced the morning after the night before. Abbreviated to AGB.
2006		www.abc.net.au: Dr Karl’s Q&A forum 16 Feb. 🌐 A good way to expain the soft poo hard poo can be the good old A.G.B. (after grog bog). after a heavy night on the booze which dehidrates [sic] the body the poo in the morning is always so good because of the high moisture content.
2017		K.M. Golland Committment [ebook] I stretched and blew off a morning thunder, knowing an After Grog Bog wasn’t too far away.