>> NZ is a small, ethnically (and therefore more importantly culturally) homogenous country

Not according to the CIA Factbook it isn't:

NZ: European 71.2%, Maori 14.1%, Asian 11.3%, Pacific peoples 7.6%, Middle Eastern, Latin American, African 1.1%, other 1.6%, not stated or unidentified 5.4%

USA: white 72.4%, black 12.6%, Asian 4.8%, Amerindian and Alaska native 0.9%, native Hawaiian and other Pacific islander 0.2%, other 6.2%, two or more races 2.9% (2010 estimate)

I would say almost quite comparable.