Green’s Dictionary of Slang

(Aus.) excellent, first-rate.

1904		Critic (Adelaide) 25 May 4/3: T’other day Adelaide ’Tiser published nearly two columns of correspondence from well-meaning, but exceedingly dreary persons who argued at great length about how many times the word bontosher occurred in Holy Scripture, or something to that effect.
1912		Sun (Kalgoorlie, WA) 19 May 7/4: I’ll go and be sailor. It’s a bontoshter life.
1929		Sydney Morn. Herald 22 July 6/2: The other slang terms ‘bonzer,’ etc., are of French derivation, and crept into the language at least thirty years ago. The original slang term was ‘bontosher,’ from the French ‘bon toujours’ (good always), and in course of time became ‘bontosher,’‘boshter,’‘bonza’ or ‘bonzer,’ and, finally, bosca’.
1942		Dly Teleg. (Sydney) 27 July 4/6: Bonzer first appeared in the mid-19th century as bon or bons, probably from the Scottish bonny or French bon. In a 100 years many forms developed: Bonter, bontager, bontogerino, bontosher.
2004		J. Lambert Macquarie Aus. Sl. Dict. 24/1: bontosher: obsolete term equivalent to bonzer (see entry).