Wind-blown from the east
New Zealand lies in the path of eastward-flowing currents, which are driven by winds that blow across the South Pacific Ocean. These winds – the south-east trades to the north, and the roaring forties to the south – drive water along the equator, down Australia’s east coast in the East Australian Current, and across the Tasman Sea. The flow splits around the western side of New Zealand and joins up again near the Chatham Rise, east of the South Island. Bathed by relatively warm water from the subtropics, New Zealand has a temperate climate.
North Island currents
The East Auckland Current flows south-east along the north-east coast of the North Island. The current travels at speeds up to 50 centimetres per second. The origins of these waters are tropical; occasionally tropical-reef fish are found at the Poor Knights Islands. Typical surface temperatures at the current’s northern reaches are 20–22°C in summer, and 15–16°C in winter.
Part of the East Auckland Current continues south, where it becomes the East Cape Current. When this current encounters the Chatham Rise, it is forced offshore and flows eastwards along the rise. Inshore, the Wairarapa Coastal Current flows north-east along the Wairarapa coast, bringing relatively cool water to the region. As this water moves up the coast, some gets pulled into the East Cape Current, so that the Wairarapa Coastal Current probably does not extend north of Māhia Peninsula. Temperatures in the Wairarapa Coastal Current are 1–2°C cooler than in the East Cape Current.
Several permanent or nearly-permanent eddies (circular currents) have been identified around New Zealand, such as the North Cape and East Cape eddies and the Wairarapa Eddy. Permanent eddies tend to arise where the sea floor forces currents to make large changes in direction.
Currents to the west of New Zealand are weaker and more variable than those along the east. The West Auckland Current flows southwards along the west coast of the North Island from North Cape to Raglan, where it is met by north-flowing currents in the North Taranaki Bight. In the South Taranaki Bight, the D'Urville Current flows south-east and through Cook Strait.
South Island currents
The Southland Current is the main current along the east of the South Island. It flows north-eastwards past Stewart Island and along the Otago coast, reaching speeds of 25 centimetres per second, and extending 130 kilometres offshore. At the Chatham Rise, it veers east to become part of the subtropical gyre (giant circular current on the surface of the ocean).
On the South Island’s West Coast, the Westland Current flows north until it reaches the south Taranaki Bight, where it contributes to the D'Urville Current.
South of New Zealand, the Southern Ocean’s westerly winds drive the Antarctic Circumpolar Current, which flows continuously around the globe. This is the world's strongest ocean current, reaching down 4 kilometres to the ocean floor and transporting about 100 times the volume of water of all the world’s rivers. The current does not directly affect New Zealand’s main islands. However, the Campbell Plateau to the south deflects the current south and channels it north past the Antipodes Islands before the flow resumes its eastward course. Further south, cold, downward-moving winds, known as katabatic winds, flow off Antarctica. These winds drive a westward current and form a clockwise gyre in the Ross Sea.