Tides around New Zealand

All around the New Zealand coastline the tidal regime is semi-diurnal. This means that on most days two high and two low tides will occur at any given location. However, if the first tide for the day occurs after about 5:10 am there will be just three tides on that day – the fourth one in the sequence will take place very early the following day.

In general terms, New Zealand's tides can be grouped into two categories according to their location on either the west or east coast. The distinguishing features are the ranges and the frequencies of the spring/neap cycle as shown in the following table.

The following diagrams of predicted tide curves at Port Taranaki (west coast) and Napier (east coast) illustrate these characteristics. Note that both tide curves are plotted using the same vertical scale. Port Taranaki exhibits the classic fortnightly spring/neap cycle caused by the changing alignment of the Earth-Moon-Sun. Note, too, that the range of the mid-month spring tide is not the same as the preceding and following spring tides. Large spring tides occur a few days after new Moon when the gravitational forces of the Moon and Sun are acting in the same direction, whereas, smaller spring tides occur after full Moon when the Moon and Sun are on opposite sides of the Earth. At Napier, the monthly spring/neap cycle is a result of the dominant effect of the lunar distance and is known as the perigean tide.

The diurnal inequality, where successive high or low water heights are not the same, is also apparent in the diagrams. This phenomenon is particularly evident in the Port Taranaki example for high waters during the periods March 8-16 and 22-30.

The tidal bulge travels around New Zealand's coastline in an anti-clockwise direction. This is contrary to what is expected from the Coriolis Effect in the southern hemisphere. Regions surrounded by amphidrome points experience a counter-rotating circulation; New Zealand is situated in such a location which explains why the tidal bulge travels around the country in an anti-clockwise direction.

This tidal bulge takes approximately 13 hours to travel around the North and South Islands. Therefore, at any one given time there is always a high tide (and a low tide) occurring somewhere around the coast. This pattern of flow is the reason why the state of the tides in Auckland's Waitemata and Manukau Harbours are never the same despite their close proximity. The tide has to travel from Auckland around Northland to reach the Manukau Harbour – a journey that takes about 3½ hours on average.

Since the Moon is the principle force driving New Zealand's semi-diurnal tides, a complete tide cycle lasts on average 12 hr 25 min. Each incoming (flood) and outgoing (ebb) tide could, therefore, be expected to last about 6 hr 10 min. This is indeed the case at most of the standard ports – with Dunedin and Picton being notable exceptions. At Dunedin the flood interval is consistently 30 minutes shorter, and the ebb tide correspondingly longer, than the nominal interval. At Picton the variation is more pronounced. The duration of the incoming tide has a range of 5½ – 9½ hours and ebb tides have a range of 4½ – 8½ hours. The longest durations are observed during the neap tides that occur 3 – 4 days after first quarter Moon in February, March, August and September. This appears to be due to the effect that the sizes of the semi- and quarter-diurnal tidal constituents have on the relatively small neap tide range at Picton.

Cook Strait: Between the North and South Islands the tide travelling northward along the east coast passes the southbound tide to the west. The strong difference between the states of these tides, combined with the east/west tide height difference, produces the complex pattern of strong currents that flow through Cook Strait.