Green’s Dictionary of Slang

first-rate, excellent, also used adv., very well (see cite 1907).

1907		Truth (Brisbane) 1 Sept. 3/5: [He] was taken in hand by the Army, provided with a temporary home, food, etc., and later on placed securely in a job, with a £50 guarantee; where he is understood to be doing ‘bonserino’.
1910		Timaru Herald 14 Jan. 3: I don’t figure out where ‘bashter’ [sic] comes from. Is it ‘boshter’ or ‘bosker’? Oh, ‘bonza,’ is it? [...] Is ‘bonzarino’ the same thing? Oh, it’s the feminine, it is? Sure, I thought it was the same. Well, that means ‘swell’ or ‘dandy’ or ‘great’ or ‘fine’.
1911		Sydney Sportsman 9 Aug. 6/3: His ‘clobber’ and diamonds, by all accounts, were real ‘bonzerino’.
1913		Truth (Perth) 1 Nov. 12/4: There are some 140 pages throughout which are depicted the latest fashions in frocks […] unique underskirts, glad gloves, bonzerino blouses, and a thousand and one other articles of luxurious upholstery for the female form divine.
1929		Gilgandra Wkly (NSW) 21 Mar. 5/3: Through the medium of your grey ’eaded kobber Mick, I received one bonserino packit of the pink fellah callum prawn.
1932		Gundagai Indep. (NSW) 18 Apr. 2/4: The Berremangra ladies, who know how to cook, prepared a ‘bonzerino’ supper.