Green’s Dictionary of Slang

excellent, first-rate.

1918		Journal (Adelaide) 16 Jan. 1/6: ‘What else’ve ye got, Bill?’‘Kidney pudden, plum pudden, tin of custard, scags, pair of bonzo socks’.
1932		Methodist (Sydney) 14 May 16/2: We had a bonzo water-melon yesterday off our vine, but I assure you, Uncle, it didn’t keep very long!
1941		S.J. Baker Pop. Dict. Aus. Sl. 12: BONZERINO, BONZIORIE, BONZO: As for ‘bonzer.’.
1953		Nthn. Standard (Darwin) 22 Oct. 4/3: To serve this dainty morsel we had Ted Kingston, a ‘bonzo’ type and as assistants Jim Wishart and Graham Smith.