Green’s Dictionary of Slang

1. a member of the 8th Infantry Brigade of the Australian Imperial Forces (A.I.F.) who arrived in Egypt too late to join in the Gallipoli campaign.

1914		Sport (Adelaide) 13 Nov. 5/5: Olly Mc. looked well walking arm in arm with Harry H. the chocolate soldier.
1915		T. Skeyhill ‘Me Brother Wot Stayed at ’Ome’ Soldier Songs from Anzac (1916) 28: But ’e called me a chocolate soldier, / A six-bob-a-day tourist, too. / ’E says,‘You’ll not reach the trenches; / Nor even get a view’.
1918		R.H. Knyvett ‘Over There’ with the Australians 133: There was a good deal of rivalry between us and another brigade known as ‘The Chocolate Soldiers’. They received this nickname because they were the most completely equipped unit that ever left Australia.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: chocs. The 8th Brigade, A.I.F. (‘Tivey’s chocolate soldiers.’).

2. a soldier who was drafted into the WWII militia but never left Australia.

1943		A. Dawes Soldier Superb 83: ‘Chocko’ abbreviates ‘chocolate soldier’, formerly a term of opprobrium rather than affection applied by men of the A.I.F. to ‘Saturday soldiers’ – militiamen called up for home defence [AND].
1979		Southerly iv 368: He’s in the University Regiment. He’s a chocolate soldier [AND].