Green’s Dictionary of Slang

(Aus./N.Z.) an outside privy.

1945		N. Lindsay Cousin from Fiji 14: I will say Grandma’s pretty good sport, locking herself in dumpties and blurting out all that hot stuff at dinner.
1965		R.H. Conquest Horses in Kitchen 85: Find yourself a shady spot behind the dumpty-doo [...] and do some hard thinking.
1966		P. White Solid Mandala (1976) 296: A few steps across the yard to the dumpty.
1970		N. Beagley Up and Down Under 98: The ‘Dumpty’ is usually down the garden path in the backyard.