Green’s Dictionary of Slang

(Aus.) 50 strokes of the lash.

1853		J. Lang Botany Bay 40: By the way, there were slang terms applied to these doses of the lash: twenty-five was called a ‘tester’; fifty, a ‘bob’; seventy-five, a ‘bull’; and a hundred a ‘canary.’.
1906		(con. early 19C) Cobargo Chron. (NSW) 2 Nov. 4/2: [as 1853].
1945		Baker Aus. Lang. 44: Nor, though they are formed on English slang terms for coins, are [recorded] [...] bob, fifty lashes.