The Department of Internal Affairs (DIA; Māori: Te Tari Taiwhenua) is the public service department of New Zealand charged with issuing passports; administering applications for citizenship and lottery grants; enforcing censorship and gambling laws; registering births, deaths, marriages and civil unions; supplying support services to ministers; and advising the government on a range of relevant policies and issues.

Other early functions included inspecting sheep, running prisons, supervising government printing, licensing auctioneers, registering births, deaths and marriages, collecting statistics, and responsibility for gambling, fire brigades, constitutional matters (including running elections) and citizenship.

[3] Over the twentieth century the department's functions would include cultural affairs, civil defence, a translation service, conservation, tourism, sport and recreation, support for ethnic communities, and support services for government ministers.

[14][15] The Office for the Community and Voluntary Sector was transferred to the department from the Ministry of Social Development in 2011.

[16] In 2021, the department gained the legal ability to process requests from people wishing to change their sex on their birth certificate, including to a non-binary marker, rather than them needing to go through the court system.

[17][18][19][19] In mid-April 2024, the department experienced a backlog in processing New Zealand passport applications due to the installation of computer system upgrades in March 2024 and increased seasonal demand.