Statistics New Zealand (StatNZ) defines metropolitan areas by reference to "urban area"-type localities.  Although Moody's Analytics prefers to use official definitions, StatNZ publishes insufficient time series data at this level of geography; therefore, we have elected to define our own metro areas by reference to "region"-type localities.

Conveniently, the urban areas constitute the majority of the population in their respective regions.  Per the 2010 census (Excel file attached):

•  Auckland Urban Area = 93% of Auckland Region
•  Wellington Urban Area = 81% of Wellington Region
•  Christchurch Urban Area = 69% of Canterbury Region

References

• StatNZ: Subnational population estimates
• Moody's Analytics: Official geographic structure of New Zealand
• Moody's Analytics: Subnational data summary for New Zealand
• Moody's Analytics: Estimated data for New Zealand

Table 1 of 1. Metropolitan areas in New Zealand. We define each of our metropolitan areas as conterminous with a region. 

New Zealand has a national geo code of INZL, all codes for its geographic subdivisions use INZL as a prefix, and all Moody's Analytics metro areas feature the letter "M." Each of the three areas is identified as INZL(underscore)M(letter)(letter)(letter), and may be retrieved en masse by using the basket geo wild card INZL_M^^^.

• Census Population - 2006-2010 (including urban areas).xls