The Royal New Zealand Navy (RNZN; Māori: Te Taua Moana o Aotearoa, lit. 'Sea Warriors of New Zealand') is the maritime arm of the New Zealand Defence Force. The fleet currently consists of eight ships. The Navy had its origins in the Naval Defence Act 1913, and the subsequent acquisition of the cruiser HMS Philomel, which by 1921 had been moored in Auckland as a training ship. A slow buildup occurred during the interwar period, and then in December 1939 HMS Achilles fought alongside two other Royal Navy cruisers at the Battle of the River Plate against the German ship, Graf Spee.

The first recorded maritime combat activity in New Zealand occurred off the northern tip of the South Island in December 1642. Māori in war canoes attacked and killed four seamen from Abel Tasman's party, who were at the time in low boats between the main ships.

The New Zealand Navy did not exist as a separate military force until 1941. The association of the Royal Navy with New Zealand began with the arrival of James Cook in 1769, who completed two subsequent journeys to New Zealand in 1773 and 1777. Occasional visits by Royal Navy ships were made from the late 18th century. The Royal Navy played a part in the New Zealand Wars: for example, a gunboat shelled fortified Māori pā from the Waikato River in order to defeat the Māori King Movement.

In 1909 the New Zealand government decided to fund the purchase of the battlecruiser HMS New Zealand for the Royal Navy, which saw action throughout World War I in Europe. The passing of the Naval Defence Act 1913 created the New Zealand Naval Forces, still a part of the Royal Navy. The first ship given by the British government for the New Zealand Naval Forces was the cruiser HMS Philomel, which escorted New Zealand land forces to occupy the German colony of Samoa in 1914. Philomel saw further action under the command of the Royal Navy in the Mediterranean, the Red Sea, and the Persian Gulf. By 1917 she was worn out and was sent back to New Zealand where she served as a depot ship in Wellington Harbour for minesweepers. In 1921 she was transferred to Auckland for use as a training ship.

The New Zealand Naval Forces passed to the control of Commander-in-Chief, China, after the Royal Navy forces in Australia came under Canberra's control in 1911. From 1921 to 1941 the force was known as the New Zealand Division of the Royal Navy. The cruiser Chatham along with the sloop Veronica arrived in 1920, Philomel was transferred to the Division in 1921, as was the sloop Torch, HMS Laburnum arrived in 1922 and then HMS Dunedin in 1924. HMS Diomede and the minesweeper HMS Wakakura arrived in 1926. Between World War I and World War II, the New Zealand Division operated a total of 14 ships, including the cruisers HMS Achilles (joined 31 March 1937) and HMS Leander, which replaced Diomede and Dunedin (replaced by Leander in 1937).

When Britain went to war against Germany in 1939, New Zealand officially declared war at the same time, backdated to 9.30 pm on 3 September local time. But the gathering in Parliament in Carl Berendsen's room (including Peter Fraser) could not follow Chamberlain's words because of static on the shortwave and waited until the Admiralty notified the fleet that war had broken out before Cabinet approved the declaration of war (the official telegram from Britain was delayed and arrived just before midnight).

HMS Achilles participated in the first major naval battle of World War II, the Battle of the River Plate off the River Plate estuary between Argentina and Uruguay, in December 1939. Achilles and two other cruisers, HMS Ajax and HMS Exeter, severely damaged the German pocket battleship Admiral Graf Spee. The German Captain Hans Langsdorff then scuttled Graf Spee rather than face the loss of many more German seamen's lives.

Achilles moved to the Pacific, and was working with the United States Navy (USN) when damaged by a Japanese bomb off New Georgia. Following repair, she served alongside the British Pacific Fleet until the war's end.