More links and websites
The site of the New Zealand Dictionary Centre, which researches New Zealand English and its usage.
More suggestions and sources
Baker, Sidney J. New Zealand slang: a dictionary of colloquialisms. Christchurch: Whitcombe & Tombs, 1941.
Bardsley, Dianne. ‘The rural New Zealand English lexicon 1842–2002.’ PhD diss., Victoria University of Wellington, 2003.
Orsman, H. W., ed. The dictionary of New Zealand English: a dictionary of New Zealandisms on historical principles. Auckland: Oxford University Press, 1997.