Green’s Dictionary of Slang

(Aus.) a small stone, a pebble.

1941		Baker Popular Dict. Aus. Sl. 84: Yonnie, a small stone, a pebble.
1952		Riverine Herald (Echuca, Vic.; Moama, NSW) 8 Sept. 4/5: At and near the town’s new playground citizens tread in fear of being ‘clocked with a yonnie’ (hit by a stone).
1988		Canberra Times (ACT) 4 Dec. 21/1: No street-wise tough in him. Skinny and timid, I suppose. He didn’t go around chucking yonnies through people’s windows or even stealing berries from neighbour’s backyards.
1994		Canberra Times (ACT) 7 Aug. 2/1: When I was a schoolboy, I remember stones suitable for throwing being referred to as ‘yonnies’ and ‘brinnies’ - the former having made it into the Macquarie, the latter presumably extinct.
2005		M.B. ‘Chopper’ Read Chopper 4 265: Working her way through a bag of chips and watching us toss yonnies at the headmaster’s window.