Green’s Dictionary of Slang

(Aus.) an admirable individual, a ‘great bloke’.

1916		Sport (Adelaide) 7 July 4/7: You are a bonzo, Konk A., going to Quorn races and getting ‘full.’ Turn it up, Konk.
1945		S.J. Baker Aus. Lang. 126: [There are] two variations that have come into use in recent years, bonziorie and bonzo.