Green’s Dictionary of Slang

(Aus.) a general form of address to a man or boy.

1946		D. Stivens Courtship of Uncle Henry 112: I’ll fix it up, sonno.
1955		N. Pulliam I Travelled a Lonely Land (1957) 134: Here, sonno. (This isn’t profanity, just their way of saying ‘Pal’ or ‘Sonny’.).
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 171: ‘Well, sonno,’ said Uncle Ern, stoking up the briar.