Green’s Dictionary of Slang

(Aus./N.Z.) first-rate, excellent, attractive.

1941		Baker N.Z. Sl. 51: Of children’s terms [...] we may note snitcher, snitch, [etc.] [...] descriptive of something superlative or excellent, both as nouns and adjectives.
1941		Baker Popular Dict. Aus. Sl.
1955		B.J. Cameron Collection n.p.: (TS July) snitcher (adj) Marvellous [DNZE].