Tidal currents are the horizontal movements of water which accompany the rhythmic rise and fall of the tide. At each place the phase of the tidal current has a definite relationship to the phase of the tide, but the phase relationship varies from place to place. Thus the tidal currents do not necessarily change direction at the time of high or low water. Indeed, this seldom occurs on the New Zealand coast, and the current may flow at its maximum velocity at the times of high and low water. Off the open coast tidal currents do not attain high velocities, but strong tidal currents are found between Three Kings Islands and the northern tip of North Island, in Cook Strait, and in Foveaux Strait.
Tidal currents in Cook Strait are particularly interesting. High water on the North Island side of the strait occurs five hours before high water on the opposite side, which means that when it is high tide on one side it is almost low tide on the other. This difference in sea level across the strait gives rise to strong tidal currents. The speed and duration of these currents are made quite variable by the heavy gales which are often encountered in this area and by other meteorological conditions. Particularly strong tidal currents which reach velocities of 5 to 6 knots are found off Cape Terawhiti. Fast-flowing tidal currents are also encountered in French Pass, a narrow channel leading into Tasman Bay between D'Urville Island and the mainland of South Island. The currents here reach speeds up to 7 knots, and the water flowing through the Pass gives the impression of a fast-flowing river.
by Norman MacKillop Ridgeway, New Zealand Oceanographic Institute, Wellington.