Green’s Dictionary of Slang

working bullock n.

In phrases

arse like a working bullock (also backside like...)

[2000s] (N.Z.) having large buttocks.

like a working bullock (adj.)

1. [mid-19C-1950s] (Aus.) used of unsuccessful racehorses, an image of ponderous sloth.

2. [late 19C+] (Aus.) strong and dutiful if inelegant.