Green’s Dictionary of Slang

a stone-thrower.

1900		Bulletin (Sydney) 24 Mar. 12/4: Some people say our national forte is throwing by hand, and point to the skill of Australian bowlers, black boomerangers, and larrikin ‘rockers.’.
1996		(con. 1950s) C. Kenneally Maura’s Boy 46: Past the Croppy Boy and add a ‘rocker’ to its mound of stones.