Green’s Dictionary of Slang

(Aus./N.Z.) to assist.

1928		V. Palmer Passage 21: How quick the boy was in picking up anything when he got a half hint! And what a wonderful off-sider he would be in a year or two. [Ibid.] 41: A boy like Hughie might as well climb into his coffin right away as go off-siding for old Kunkel.
1996		B. Scott Banshee and Bullocky 10: Arch went offsiding for a bullocky.