Alex replied:

G'day, maybe its from the old recipe for cooking donkeys? For those who don't remember, you get a big pot of boiling water and a large rock. Add the donkey and the rock to the water, cook until the rock is soft, throw away the donkey and eat the rock. Cheers, Alex.

neil replied:

Ray, I'm not sure but I heard once that the term refers to the old mining days early 1900's when donkeys and pit ponies were often used to do the hard yakka in boiler and mining maintenance. John Simpson Kirkpatrick the Anzac legend, worked donkeys in the mines around the Coledale/Bulli area pre-war. Sounded pretty good to me ?