Green’s Dictionary of Slang

(Aus.) urination.

1960		J. Franklyn Dict. of Rhy. Sl. 84/1: Jerry Riddle Piddle [...] an alternative form to the more popular Jimmie.
1971		F.J. Hardy Outcasts of Foolgarah (1975) 34: ‘I want to go for a Gerry Riddle and an Edgar Britt.’ [...] ‘A what?’ ‘A piddle and a shit.’.
2002		Pete’s Aussie Sl. Home Page 🌐 Gerry Riddle: a piddle, to urinate.