Green’s Dictionary of Slang

(NZ) good, first-class.

1907		Maitland Mercury (NSW) 11 Jan. 2/3: Other expressions dotted over the pages are: ‘A snorter,’ ‘not half bad,’ ‘not so rusty,’ ‘bosko,’ ‘bosca,’ ‘O.K.,’ ‘A1,’ [etc].
1907		Eve. Post (Wellington) 19 Jan. 3: That ugly word ‘bosca,’ which is not yet accepted by professors of English, and has yet no definite form, appears in several guises - bosko, boshker, boshta, bosca.
1913		Nelson Eve. Mail 16 Oct. 6: He say[s] not, ‘Will you have a cigar; but will you ‘sample’ my cigar. They’re bosko, which means good’.