The Waitangi sheet from the Treaty of Waitangi

The British Government appointed James Busby as British Resident to New Zealand in 1832 and in 1835, following an announcement of impending French settlement by Charles de Thierry, the nebulous United Tribes of New Zealand sent a Declaration of the Independence to King William IV of the United Kingdom asking for protection. Ongoing unrest and the dubious legal standing of the Declaration of Independence prompted the Colonial Office to send Captain William Hobson to claim sovereignty for the British Crown and negotiate a treaty with the Māori. The Treaty of Waitangi was first signed in the Bay of Islands on 6 February 1840. In response to the commercially run New Zealand Company's attempts to establish an independent settlement in Wellington and French settlers "purchasing" land in Akaroa, Hobson declared British sovereignty over all of New Zealand on 21 May 1840, even though copies of the Treaty were still circulating. With the signing of the Treaty and declaration of sovereignty the number of immigrants, particularly from the United Kingdom, began to increase.

New Zealand, originally part of the colony of New South Wales, became a separate Colony of New Zealand on 1 July 1841. The colony gained a representative government in 1852 and the 1st New Zealand Parliament met in 1854. In 1856 the colony effectively became self-governing, gaining responsibility over all domestic matters other than native policy. (Control over native policy was granted in the mid-1860s.) Following concerns that the South Island might form a separate colony, premier Alfred Domett moved a resolution to transfer the capital from Auckland to a locality near the Cook Strait. Wellington was chosen for its harbor and central location, with parliament officially sitting there for the first time in 1865. As immigrant numbers increased, conflicts over land led to the New Zealand Wars of the 1860s and 1870s, resulting in the loss and confiscation of much Māori land. In 1893 the country became the first nation in the world to grant all women the right to vote and in 1894 pioneered the adoption of compulsory arbitration between employers and unions.

In 1907, at the request of the New Zealand Parliament, King Edward VII proclaimed New Zealand a dominion within the British Empire, reflecting its self-governing status. In 1947 the country adopted the Statute of Westminster, confirming that the British parliament could no longer legislate for New Zealand without the consent of New Zealand. New Zealand was involved in world affairs, fighting alongside the British Empire in the First and Second World Wars and suffering through the Great Depression. The depression led to the election of the first Labor government and the establishment of a comprehensive welfare state and a protectionist economy. New Zealand experienced increasing prosperity following World War II and Māori began to leave their traditional rural life and move to the cities in search of work. A Māori protest movement developed, which criticised Eurocentrism and worked for greater recognition of Māori culture and the Treaty of Waitangi. In 1975, a Waitangi Tribunal was set up to investigate alleged breaches of the Treaty, and it was enabled to investigate historic grievances in 1985. The government has negotiated settlements of these grievances with many iwi, although Māori claims to the foreshore and seabed have proved controversial in the 2000s.