Statistics New Zealand provides many free sources of statistical data.
Census (1996 - 2018)
figureNZ - a trust which cumulates NZ statistics from any sources
Pātaka Raraunga - Data for and about Māori
Pacific Data Fale o Aotearoa - Data for and about Pacific peoples
New Zealand Covid Statistics
19th century Aotearoa New Zealand statistical publications
Statistical publications Aotearoa New Zealand 1840-2000
Statistics of the Dominion of New Zealand 1907-1920
Yearbook Collection 1893-2012