Green’s Dictionary of Slang

underground mutton n.

(Aus.) rabbit.

1904		Advertiser (Adelaide) 14 Dec. 5/5: Those [iguanas] that infest the haunts of the underground mutton (rabbits) are not destroyed, but the hen-egg robbers are dispatched without mercy.
1915		Dubbo Liberal (NSW) 15 June 3/3: ‘The only alternative,’ says Mr Blacket, ‘will be underground mutton [...] and many who have despised bunny as food will be compelled to take to it.
1925		Townsville Daily Bull. (Qld) 27 June 12/6: This charioteeer of deceased underground mutton merely murmered [...] ‘run away and take your box of stinks with yer’.
1936		Advertiser (Adelaide) 26 May 6/5: We used to sit on stumps [...] and eat our dinners — underground mutton (rabbit) pasties often.
1945		Mail (Adelaide) 22 Dec. 4/3: A few decades back an indsicreet resident imported rabbits with the idea of making underground mutton available.
1946		W.S. Hughes Slaves of the Samurai 145: [note] ‘Underground Chicken.’ No connection with the Black Market or outside help, but merely the Australian facetious name for rabbit.
1953		Cairns Post (Qld) 22 July 5/1: Myxamatosis, a disease to which rabbits quickly succumb, reduced the population of ‘underground mutton’.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xliii 11/2: underground mutton: Rabbit.
1975		(con. 1930s) H. Anderson Men of the Milford Road 33: There was no shortage of meat as long as you liked ‘underground mutton.’.
1976		Aus. Women’s Wkly 19 May 22/2: He is flogging ‘fresh underground mutton’.
1986		R. Beckett Dinkum Aussie Dict. 54: Underground chicken: Rabbit. In recent years this phrase has been mistranslated as ‘underground mutton.’.
1988		McGill Dict. of Kiwi Sl. 117/1: underground mutton rabbit.
1999		(con. 1880s) C. Skinner Spurgeon & Son 53: Tom saw several of their carts full of freshly killed bush rabbits [...] this ’underground chicken,’ when slowly baked in a wood oven, tasted indeed as succulent as poultry.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].
2010		R. Kirkwood Variant Breed 252: One species of animal surviving in this modern day though, and thriving I may add, (even populating the area heavily, despite all the snakes) was underground mutton.