A shallow, well-mixed layer, approximately 50 to 200 metres thick, forms the surface layer of the oceans, the mixing resulting from the effect of wind and waves. The vertical differences of temperature and salinity within this layer are very small but the water properties can change fairly rapidly because the layer is subjected to solar radiation, evaporation, and precipitation. At the bottom of this layer there is a region where the vertical temperatures decrease rapidly over a small depth, with consequent sharp changes in the density of the water. This region is called the thermocline and mixing cannot proceed rapidly there because of the steep density gradient which exists. As a result, water properties such as temperature and salinity do not change quickly below the thermocline. These conservative properties are used to classify oceanic water into various “water masses”, and the movement of these water masses can be traced over long distances. This fact is used as a subsidiary method of determining currents. Different water masses exist at different depths and a vertical section may consist of an upper, intermediate, deep, and bottom water mass.

Two distinct upper water masses are present in the immediate vicinity of New Zealand, the Sub-Antarctic Water Mass lying to the south and the Subtropical Western South Pacific Water Mass lying to the north. These two water masses meet in a region called the Subtropical Convergence Region which is often (but not always) characterised by comparatively sharp changes in temperature and salinity between the warmer, more saline subtropical water and the cooler, less saline Sub-Antarctic Water.

The Sub-Antarctic Water Mass extends southwards to somewhere between about 54° s and 62 s where another major boundary, the Antarctic Convergence, separates the Sub-Antarctic Water from the colder Antarctic Water. The Sub-Antarctic Water Mass thus lies in the westerly wind belt of the so-called Roaring Forties and Fifties, and the main movement of this water is towards the north-east. This movement is called the West Wind Drift.

The subtropical waters move mainly westwards under the influence of the south-east trade winds and this movement is called the Trade Wind Drift. The Australian continent bars the westward movement of part of this Trade Wind Drift and the water is deflected to move southwards off the east coast, thus forming the East Australian Current. When the subtropical water transported by this current meets the north-east-moving Sub-Antarctic Water, it turns and moves eastwards across the Tasman Sea as the Tasman Current.