The Mega Tsunami That Hit Eastern Australia: Lord Howe

When the Ocean Rose: The Mega-Tsunami of Lord Howe Island

Picture this: you’re standing on a lonely volcanic island, hundreds of kilometres out in the Tasman Sea. Around you, cliffs rise sheer from turquoise water, covered in lush palms and tangled rainforest. The waves slap rhythmically against the rocks, harmless and familiar. It feels timeless, unchanging, like the ocean has always lapped at these shores in just this way.

But the rocks tell a different story. Scattered along parts of the coastline of Lord Howe Island are clues that, sometime in the last millennium, something utterly extraordinary happened here. Evidence points to a colossal wave — a mega-tsunami — that smashed into the island with unimaginable force, stripping vegetation, tearing up bedrock, and hurling debris far inland and high above the sea.

This isn’t the kind of tsunami most of us imagine, the tragic but more familiar earthquake-triggered walls of water we’ve seen on the news in places like Indonesia or Japan. What scientists suggest hit Lord Howe Island was much rarer, much more violent, and far bigger than anything in human memory. The evidence preserved in stone and sediment hints at a single, cataclysmic event, one so powerful it re-shaped entire stretches of the island’s coast.

A little over a year ago I released a few videos that covered the mega tsunamis that hit eastern Australia. All of the information is based off the study: Cosmogenic mega-tsunami in the Australia region: are they supported by Aboriginal and Maori legends?

The links to the other videos and the study will be in the description. This is the final episode of the series. Where we look into what evidence the study says exists at Lord Howe Island.

A Tiny Island in a Vast Ocean

First, let’s get our bearings. Lord Howe Island is a small, crescent-shaped volcanic remnant that sits about 600 kilometres east of the New South Wales coast. It rises steeply from the seafloor, part of a long chain of extinct volcanoes that trail across the Tasman Sea. Today it’s best known as a lush, subtropical paradise — home to giant stick insects, rare birds, and the southernmost true coral reef in the world.

But because it’s isolated, it also makes a perfect natural recorder of big oceanic events. Unlike the mainland, which has rivers and tides and constant human disturbance, Lord Howe has stretches of rocky coastline where powerful waves can leave scars that linger for centuries. And those scars are exactly what drew scientists’ attention in the late 20th century.

The Clues in the Rocks

Starting in the late 1980s and early 1990s, Australian researchers began surveying the east coast and nearby islands for signs of unusual wave action. They weren’t just looking at sand dunes or beach ridges — they were paying close attention to bedrock sculpturing.

On Lord Howe, they found wave-cut features far higher than any storm surge could reach. Huge blocks of rock had been quarried from the shoreline and tossed inland. Piles of imbricated boulders — massive stones stacked and tilted in the same direction — lay on headlands, testifying to water powerful enough to lift and flip them like pebbles. Some of these deposits sit dozens of meters above the modern sea level, in positions where ordinary waves, or even cyclones, could never have delivered them.

In short, the island carried the fingerprints of an ocean event on a scale that defies day-to-day experience.

How Big Was It?

So just how tall was this wave?

The evidence suggests that the tsunami on Lord Howe Island had a run-up height — that’s how high the water reached above normal sea level — of more than 100 meters in some places. Let that sink in. That’s taller than a 30-story building, taller than the Statue of Liberty, taller than the Sydney Harbour Bridge.

This wasn’t just a “big wave.” It was an ocean on the move.

When researchers traced the distribution of eroded cliffs and displaced blocks, they calculated that the tsunami must have overtopped rocky headlands, surged inland, and then plunged back into the sea with enough violence to leave thick deposits of sand and gravel behind. At certain sites, sediments were found up to 130 meters above sea level — deposits of marine origin sitting far higher than any normal coastal process could explain.

To put it simply: whatever happened here, it dwarfed even the worst storm waves.

*Mega Tsunami Depiction

The Violence of the Impact

To appreciate the violence of such an event, think about the mechanics of water. Normal waves, even storm waves, are surface disturbances. They slap, they erode, they wash away sand. But a tsunami isn’t a surface ripple — it’s the entire body of water in motion, from the seabed up.

When that mass of water slammed into Lord Howe Island, the energy release would have been comparable to a continental-scale flood. Whole slabs of basalt could have been ripped from the cliffs, some weighing many tonnes, and hurled inland. Boulder fields that today look like they were dumped by giants may have been left behind by a single surge. The over-washing waves would have chewed into headlands, leaving smooth ramps and knife-like grooves cut into rock faces.

And then came the backwash. Water rushing seaward would have dragged debris with it, re-depositing sands and gravels in new layers. These sudden pulses of erosion and deposition are the hallmarks geologists look for when identifying tsunami impacts, and on Lord Howe, they’re abundant.

A Regional Disaster

The striking thing about Lord Howe Island is that it wasn’t alone. Around the same time — roughly the 15th century — similar evidence of catastrophic inundation appears along the New South Wales coast. At Bombo Headland near Kiama, for example, scientists found that a tsunami had overtopped a 40-meter cliff, dropped sediment into a sheltered bay on the far side, and left behind a chaotic jumble of coarse material.

Taken together, these findings suggested a regional-scale mega-tsunami, one that slammed into a swathe of the Tasman Sea. Lord Howe Island was simply one of the most dramatic victims, preserving the record in its volcanic rocks.

What Would It Have Looked Like?

It’s hard to truly picture an event of this scale, but let’s try.

Imagine the horizon darkening as an enormous wall of water approaches. From the shore, it wouldn’t look like a curling wave; it would look like the sea itself had risen, a vast bulge moving forward. On Lord Howe, there would have been no high ground safe enough on the eastern side.

As the tsunami struck, it would have exploded against the cliffs with thunderous force. Vegetation would have been scoured away in seconds. Birds would have been blasted from the sky, crabs and turtles hurled inland, and any low-lying ground would have been drowned instantly.

The water would have surged inland, roaring across valleys and forest, before rushing back to the ocean and tearing everything with it. When it receded, it would have left behind a raw, stripped landscape. Boulders carried far from their original positions would sit in unnatural piles. Sand and shells from the ocean floor would cap the headlands. And the island would never look quite the same again.

Why Storms Don’t Cut It

Skeptics sometimes ask: could this just have been a super-cyclone? After all, the Tasman Sea does get powerful storms.

But storms, no matter how strong, have limits. Wind-driven waves, even from the biggest cyclones, don’t climb 100 meters up vertical rock faces. They don’t toss multi-tonne boulders onto cliff tops. They don’t leave marine sediments perched far above the highest storm surge lines.

That’s why researchers are convinced: the Lord Howe Island evidence points to something much rarer, a true outlier in Earth’s catalogue of natural disasters.

The Human Dimension

Lord Howe Island was probably uninhabited when this happened. Archaeologists have never found clear evidence of permanent human settlement there before European discovery. But on the mainland, Aboriginal communities had been living along the east coast for tens of thousands of years.

If the same tsunami that struck Lord Howe also hit New South Wales — and the geological evidence strongly suggests it did — then it would have been devastating for those coastal populations. Oral traditions preserved by Aboriginal groups along the NSW coast speak of the sea falling from the sky, or of floods that drowned whole groups of people. Whether those stories are direct memories of this event or not, they resonate with what the rocks tell us.

Imagining the Impact Today

It’s sobering to think about what would happen if such a tsunami struck again in modern times.

Lord Howe Island today is home to about 400 permanent residents, plus visitors who come for hiking, diving, and relaxation. The island is fringed by a delicate coral reef — the southernmost in the world — and supports unique ecosystems found nowhere else. A wave 100 meters high would obliterate all of it. The town, the airport, the houses, the forests, even the reefs themselves would be stripped away in moments.

And on the mainland, where millions of people now live along the New South Wales coast, the devastation would be beyond comprehension. Imagine Sydney Harbour rising like a bursting dam, or Wollongong’s beaches swallowed in minutes. Unlike a storm, there would be little warning, and the scale of destruction would be unmatched in Australia’s history.

A Catastrophe Written in Stone

Today, Lord Howe Island looks serene again. Visitors hike to the summit of Mount Gower, snorkel on the reef, and stroll along perfect white beaches. Few would ever guess that, just a few centuries ago, this place was battered by one of the largest waves Earth has ever seen.

But the boulders and sediments tell the truth. They are silent witnesses to catastrophe, reminders that even the most tranquil island can be suddenly and violently transformed.

And perhaps that’s the bigger lesson here. We humans tend to think in short timescales — a storm season, a human lifetime, maybe a century or two. But the rocks keep a deeper memory. They remind us that rare, almost unimaginable events do happen, and when they do, they leave marks that last for millennia.

Link to the study:

Cosmogenic mega-tsunami in the Australia region: are they supported by Aboriginal and Maori legends?

Here the video we made on this on the OzGeology YouTube Channel: