Green’s Dictionary of Slang

something or someone excellent, exceptional.

1904		Bulletin (Sydney) 14 Apr. 29/1: [A] bonser or bonster is comparatively superior to a bons.
1909		Newcastle Herald (WA) 8 May 5/1: Flaherty, in the words of one of the local ladies, is a ‘Bonz.’ If he knew someone thought that of him he would probably consider Newcastle a fine place to spend a holiday at some future date.
1915		Sun. Times (Perth) 7 Feb. 6/2: ‘I met such a dinky little flapper. My oath, she was a bonz!’.
1921		Sun (Sydney) 17 Oct. 6/3: This ’ere w’ite scar I wears above me lip / Was done wiv metal, Cripes! It was a bonze! / I copped it when a kid in Hogan’s Bush, / While we was scrappin’ wiv’ a rival push.