[4] The greater Cook Strait and Kaipara Harbour seem to offer the most promising sites for using underwater turbines.

Tidal power is generated by capturing some of the energy in the tides as they cycle forth and back, twice each day.

[5] Headlands and constrictions like these focus the currents, giving energy levels reaching 750 W per square metre.

There are actually two high tides, because the Earth and Moon, as a system, both rotate about a common centre of mass.

The effect of the Earth spinning about this centre is that it behaves as a centrifuge, resulting in a second high tide bulge in the ocean most distant from the Moon.

However, some of the larger harbours on the west coast of the North Island, in particular the Kaipara, experience significant currents as the tides rise and fall.

[5] The National Institute of Water and Atmospheric Research (NIWA) run a tidal computer model specific to New Zealand.

[8] The actual tide pattern and timing is determined by the nature of the resonances in each ocean basin with the various frequencies of the gravitational influences, over many cycles.

If the turbines are operated at a fixed flow aperture, the power produced is not constant but follows the tide, an effect that can be seen in the following graph.

Note that the timing follows the tides around the clock, not the usual twenty-four cycle of electricity usage.

The Opunake power station has its tailrace exiting to the beach but its operation is intermittent so if there is any tidal effect on generation there, it is unclear.

This is because the main M2 lunar tide component which circulates anti-clockwise around New Zealand is out of phase at each end of the strait.

The difference in sea level can drive tidal currents up to 2.5 metres per second (5 knots) across Cook Strait as well as into the Tory Channel.

[13] In April 2008, a resource consent was granted to Neptune Power for the installation of an experimental underwater tidal stream turbine in the strait.

Fourteen metres in diameter and constructed of carbon fibre, it will be capable of producing one megawatt.

The company has claimed there is enough tidal movement in Cook Strait to generate 12 GW of power, more than one-and-a-half times New Zealand's current requirements.

[17] On the other side of the strait, Energy Pacifica has talked for some time about applying for resource consent to install up to ten marine turbines, each able to produce up to 1.2 MW, near the Cook Strait entrance to Tory Channel.

They claim Tory Channel has tidal flows of 3.6 metres per second with good bathymetry and access to the electricity network.

[21] Crest plans to place the turbines at least 30 metres deep along a ten kilometre stretch of the main channel.

[23] However, while the Department of Conservation has approved the project, and has made substantial environmental monitoring conditions part of the consent, the project also has objectors on the grounds of claimed influences on the local ecosystems and charter fishing.

South-east trade winds dominate in the north, enlivened by an occasional cyclone from the tropics.

The rest of the country is dominated by the roaring forties, a broad band of westerly winds that span the middle latitudes of the southern hemisphere.

These winds produce some of the stormiest seas in the world, with maximum wave heights regularly exceeding 4 metres.