The incumbent[update] prime minister, Christopher Luxon, leader of the New Zealand National Party, took office on 27 November 2023.

The office exists by a long-established convention, which originated in New Zealand's former colonial power, the then United Kingdom of Great Britain and Ireland.

The convention stipulates that when the office becomes vacant, the governor-general must select as prime minister the person most likely to command the support, or confidence, of the House of Representatives.

[a] The prime minister and Cabinet are collectively accountable for their actions to the governor-general, to the House of Representatives, to their political party, and ultimately to the national electorate.

[4] Forty-two people (thirty-nine men and three women) have served as prime minister, the first of whom was Henry Sewell taking office on 7 May 1856 as premier.

[5] In making this appointment, convention requires the governor-general to act on the outcome of the electoral process and subsequent discussions between political parties by which the person who will lead the government as prime minister is identified.

[5][6] In practice, the position typically falls to an MP who is the parliamentary leader of the largest political party among those forming the government.

[11] The governor-general retains reserve powers[12] to dismiss a prime minister in certain circumstances, such as those pertaining to a no-confidence motion against the government in the House of Representatives.

Under the Constitution Act 1986, general elections are required every three years, setting the maximum period a prime minister can serve without their mandate being renewed.

[14] The office of prime minister is not defined by codified laws but by unwritten customs known as constitutional conventions which developed in Britain and which New Zealand replicated.

[7] A prime minister does hold the most senior post in government, but must also adhere to any decisions taken by Cabinet, as per the convention of collective ministerial responsibility.

[4] The written form of address for the head of government should use their full parliamentary title as applicable: The Right Honourable [name], [post-nominal letters], Prime Minister of New Zealand.

The first holder of the office, Henry Sewell, led the country for the shortest total time; his only term lasted just 13 days.

There was persistent speculation during his lifetime that Norman Kirk (Prime Minister from 1972 to 1974) was Māori and had Kāi Tahu ancestry; he never publicly identified himself as such, and there is no substantial evidence for the claim.

Use of the words prime minister as a descriptive term dates back to the First Parliament, where they are applied to James FitzGerald and Thomas Forsaith.

[46] The first person to be formally appointed to a position of executive leadership was Henry Sewell,[46] who formed a brief ministry in April 1856, at the beginning of the Second Parliament.

Despite his formal leadership role, however, his only actual title was "colonial secretary",[45] a position comparable to a minister of internal affairs.

Weld's successor, Edward Stafford, briefly changed the title to "first minister", but it was soon restored to premier during the second tenure of Fox in 1869.

Stafford met with his ministers and made decisions outside of the Executive Council (which was chaired by the governor), thus establishing the modern convention of cabinet government.

Previously, New Zealand prime ministers had attended occasional colonial and imperial conferences, but they otherwise communicated with London through the governor (a position then appointed by the British government).

[52] Constitutional conventions adopted in 1930, following the Imperial Conference held that year, increased the domestic and international prestige of the prime minister.

[52] In 1967, Keith Holyoake became the first New Zealand prime minister to select candidates for the position of governor-general without any involvement of the British government.

Holyoake advised the monarch, Queen Elizabeth II, to appoint Sir Arthur Porritt, the first New Zealand-born governor-general.

The skill of MMP management was exemplified by Helen Clark's nine years as prime minister (1999–2008), when her Labour government remained in power thanks to a range of confidence-and-supply agreements with five smaller parties.

[55] Until the premiership of Helen Clark, it was customary for senior members of the legislature, executive and judiciary—including the prime minister—to be appointed to the British Privy Council, granting them the style "Right Honourable".

On 3 August 2010, the Queen granted the prime minister, along with the governor-general, speaker of the House of Representatives and chief justice, the style "Right Honourable" upon appointment.

[57] On 21 June 2018, Labour's Jacinda Ardern became the first prime minister of New Zealand (and second elected head of government in the world) to give birth while in office.