How tides actually work
Almost everyone learns the schoolbook version of tides — the Moon pulls the ocean into a bulge, Earth rotates underneath it, two highs a day. That part is true. But it isn’t the whole story, and it doesn’t explain why some places have 16-metre tides and others have almost none, or why Wellington and Nelson are six hours out of phase even though they’re a hundred kilometres apart. Press play and we’ll build the full picture, one step at a time.
What you’ve just watched
The Moon’s gravity stretches Earth’s oceans into two bulges, one toward the Moon and one on the far side. The Earth rotates underneath them — that’s why most coasts get two high tides per day. That part is correct. Almost everyone learns it; almost no one is told the rest.
The rest: the Moon’s pull, on its own, would only raise the ocean by about half a metre. Real coastal tides go from a couple of centimetres in some places (Tahiti, parts of the Mediterranean) to sixteen metres in the Bay of Fundy. The reason is resonance. Each ocean basin oscillates at its own natural frequency — like water sloshing in a bathtub — and the Moon’s small, regular nudge sets up huge standing waves in those basins. Those standing waves rotate around fixed points called amphidromic points where the tide doesn’t actually rise or fall. Around each amphidromic point, the high-tide line sweeps like the hand of a clock, taking about 12 hours per rotation.
New Zealand sits between two of those rotating systems — one in the Tasman / Southern Ocean, one in the SW Pacific east of the country. Our east coast is on one rotation; our west coast is on the other. They’re caught at different points in their respective rotations at any given moment. That’s why the high tide reaches Wellington six hours before it reaches Nelson, even though they’re only a hundred and fifty kilometres apart.
And inside any given harbour, water doesn’t magically appear. It flows in through the harbour mouth as the local ocean tide rises, fills the slightly-lagged interior, then drains back out as the local ocean tide falls. The narrower the mouth and the bigger the harbour, the faster the water has to rush through to keep up — which is why Kaipara, Manukau and the Cook Strait approaches have the strong tidal currents they do.
The Moon, the Sun, and why we have two high tides a day
Tides are caused mainly by the Moon’s gravity, with a smaller contribution from the Sun. Because Earth and the Moon are different distances apart at different points on Earth’s surface, the side facing the Moon feels slightly more gravitational pull than the centre, and the side facing away feels slightly less. This produces two bulges — one toward the Moon, one on the far side. As Earth rotates underneath, every point on the surface passes through both bulges in roughly 25 hours, giving most coasts two high tides and two low tides a day. The 50-minute daily lag (24 hours plus 50 minutes per cycle) is because the Moon also moves a little around its own orbit each day.
This much is the standard textbook explanation, and it’s correct as far as it goes. But the bulges themselves are tiny. If the equilibrium-tide model were the whole story, every coastal tide on Earth would be about 50 centimetres high. They aren’t.
Why real tides are so much bigger than the textbook predicts
The actual mechanism that produces metre-scale coastal tides is resonance. Each ocean basin behaves like a giant bowl of water with its own natural sloshing period. The Moon provides a small, regular forcing that — if the basin’s natural period happens to match the Moon’s tidal period closely — gets amplified, sometimes massively. The classic example is the Bay of Fundy in Canada, whose geometry produces a natural sloshing period of about 12.5 hours, almost exactly the M2 tidal period. The Moon nudges, the bay rings like a bell, and the result is the largest tides on Earth — up to sixteen metres twice a day.
The standing waves set up in each basin don’t just rise and fall in place. Because of the Coriolis effect, they rotate. Each basin has one or more amphidromic points — places where, because the rotating wave passes through symmetrically, the tide doesn’t actually rise or fall at all. Around each amphidromic point, the line of high tide rotates like a clock hand, taking roughly 12 hours per full revolution. In the southern hemisphere, that rotation is counter-clockwise; in the northern hemisphere, clockwise.
The map of the world’s amphidromic systems
Oceanographers have mapped the world’s amphidromic points for the dominant M2 (lunar) tide and several smaller constituents. There are roughly a dozen of them globally for M2 alone. The Atlantic has three or four; the Pacific has several including a major one in the south Pacific east of New Zealand and another in the Tasman / Southern Ocean to the south-west; the Indian Ocean has its own. The wave patterns around them are visible in the global cotidal charts published by NOAA and various national hydrographic offices.
Our maps in this explainer use simplified versions of those published patterns. The exact positions of the amphidromic points wobble slightly with seasons and decadal climate cycles; the published values are 30-year averages.
Why New Zealand’s tides do what they do
New Zealand sits between two amphidromic systems — one rotating to our south-west in the Tasman / Southern Ocean, the other rotating to our south-east in the South Pacific. Our east coast is influenced primarily by the Pacific system; our west coast is influenced primarily by the Tasman system. The two systems are at different points in their rotations at any given moment, which is why the high tide arrives on the east coast first and the west coast about six hours later.
That 6-hour offset is what drives Cook Strait. Wellington (east coast) and Nelson (west coast) are only 150 km apart, but they’re on different sides of two different rotating waves. When Wellington is at high tide, Nelson is roughly at low tide — a height difference of around two metres can develop across the strait, and water rushes through to equalise. The result is currents up to four knots in mid-channel, much faster through specific squeezes like Karori Rip. The same logic explains the strong tidal flows through French Pass, Kaipara Heads, Manukau Heads and Foveaux Strait.
Inside a harbour
Inside any individual harbour, the tide doesn’t just appear — water has to flow in and out through the harbour mouth. The interior of a harbour lags the open ocean by 30-60 minutes typically (more for big lagged harbours like Kaipara). When the ocean is rising at the heads, water flows IN to fill the slower-rising interior. When the ocean has passed peak and is starting to fall, water keeps flowing in for a brief while because the interior is still catching up. Then as the gap closes and reverses, water starts flowing OUT through the heads. Peak current through the heads happens at mid-tide — not at high or low water, when flow is briefly slack.
This is the principle behind the live arrows on the NZ Tide Pulse map: the direction of flow through Kaipara and Manukau Heads tracks whether the tide at the heads is currently rising or falling, not whether it’s currently high or low. For straits like Cook Strait, the flow direction tracks which side has the higher water level at this moment — the side that’s higher is draining toward the side that’s lower.