Green’s Dictionary of Slang

(N.Z.) excellent! amazing!

1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] ‘If there should be any stuff up with the tickets just ask for this bloke. [...] But it should all be sweet.’ ‘Ripper’.
2003		McGill Reed Dict. of N.Z. Sl. 173: ripper! Exclamation of approval. ANZ late 1970s.