Many of New Zealand’s famous northern beaches have been left markedly eroded by a slew of severe summer storms, with one expert calling the impacts “pretty close to unprecedented”.

Satellite imagery has revealed changes in the order of five to 10m – and some coastal sites were now at their most eroded state in nearly a century.

Worst-affected spots included Northland’s Marsden Point and Ruakākā Beach, where heavy waves have turned sand dune slopes into abrupt cliff drops.

Seating at the sand dune entrance of the beach has completely disappeared, while steps appear to have moved several metres away from a once-connected path.

Ruakākā Surf Club after Cyclone Gabrielle Photo / Supplied

Whangamata and many other popular Coromandel beaches have also been hit hard.

“The erosion [around north-east coastlines] has been pretty consistent, but what we’ve seen at Whangamata’s southeast-facing beach is probably the most pronounced,” University of Auckland coastal geomorphologist Associate Professor Murray Ford said.

“Certainly, the shoreline position, on average, is a good five to 10m landward of where it was in January last year.”

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Erosion at Whangmata in the wake of Cyclone Gabrielle.

Storm surges have caused visible scarping to local foredunes running parallel to the shore, with some beach-front properties sitting metres away.

 “A lot of residents here are absolutely shocked,” said one Whangamata local, who asked not to be named.

“There used to be lots of sand dunes there, and they’ve just gone. Many people with properties along the beach won’t have been down to see what’s happened yet and they’ll get a huge shock when they do.”

It wasn’t the first time Whangamata had been hit by heavy erosion events: three years ago, big storms carved its escarpment and revealed planks that’d been buried for decades.

But this summer had been different.

“Just to have this all happen so quickly ... right from New Year, it’s just been devastating.”

Erosion at Whangamata's foredunes is among some of the "most pronounced" that researchers have observed. Photo / Supplied

A Thames-Coromandel District Council spokesperson said that, while no beaches had required closing, there’d been damage to several access structures.

“Roading infrastructure has been significantly impacted across the district, however this has mostly been due to landslips and not related to coastal erosion.”

There’d been some locations where erosion had resulted in metres of inland movement, they said, but that’d been “part of a natural process”.

“Our sand dunes have taken the brunt of the storm surge. As expected, they have absorbed the wave energy and the sand built up over the years has acted as a sacrificial wall of protection between the land and the ocean,” they said.

“When the conditions improve our dunes will recover like they have in the past. The problems are only apparent where there is property or infrastructure within the natural process area.”

In several locations, however, the council would need to “further consider” its response.

“The preference is to work with nature to allow natural rebuilding of sand dunes but it is possible in some instance, to protect roads or other infrastructure, less natural options will need to be considered.”

Elsewhere, satellite imagery has shown the dramatic before-and-after contrast of Gabrielle’s impact on at Aotea-Great Barrier Island’s Kaitoke Beach – a long stretch of golden sand home to endangered brown teal and dotterel.

These images show Kaitoke Beach, on Great Barrier Island, on February 6 and February 14, after ex-tropical cyclone Gabrielle had passed through. The images were captured by the Pléiades satellite which is operated by the CNES. Image / CNES (2023), Distribution Airbus DS.

“I’ve been looking at video footage from places like Hahei, where almost all of the sand has been scraped off the beach,” said Professor Karin Bryan, an expert in coastal processes at Waikato University.

“We’re seeing dune toes being breached and eroded, and even some of our most stable beaches like Tairua and Rings Beach are showing changes.”

In one odd case, Napier’s formerly shingle-covered Ahuriri Beach was now covered in finer sand.

Sand now stretches across Ahuriri beach. Photo / Mitchell Hageman

Bryan described this summer’s erosion as “pretty close to unprecedented” - or at least comparable to damage wrought by the historic storms of July 1978.

“That was a huge year, and was when a lot of beach monitoring programmes were set up along the coasts,” she said.

“After that, we had a very long period of stability, where people built increasingly close to the sea – and now we’re shifting back to another period of instability.”

Recovery was likely to take many years.

This image shows huge waves hitting Coromandel's Hahei Beach on February 12, as ex-tropical cyclone Gabrielle blew in. One expert says much of the beach's sand was scraped off in the event. Photo via video / Matthew Davison

“Normally, beaches will have a period of winter erosion where sand is scraped off and put offshore, before it’s gradually moved back in summer,” she said.

“But if you have these storm events all summer, which is meant to be a recovery time, there’s going to be a lot less sand ready for the winter.”

Beyond being places to swim and surf in summer, Bryan said healthy beach systems were crucial for a range of reasons.

They supported coastal ecosystems, carried immense cultural importance to iwi – sand dunes, for instance, contained urupa (ancient burial grounds) and pīngao for weaving - and buffered wave energy for houses and roads built behind them.

Researchers estimate there's been five to 10 metres of change at Whangamata beach, compared with one year ago.

“Obviously, when there isn’t much of the natural dune left, people lose their buffer zone.”

Ford added: “Those communities on Coromandel’s east coast have economies that are underpinned by healthy beaches, and people from Auckland and elsewhere around the country seek out those locations.”

He agreed it was a concern that many beaches were in poor shape ahead of winter.

“We may be moving out of La Niña, but we’re moving into winter when we do see larger swells ... so this is not a great position for them to be in.”

Ford, who, with colleagues in the collaborative Resilience to Nature’s Challenges effort has been using satellite imagery to catalogue fresh coastal impacts, also expected a lengthy recovery.

Damage to beach access near Northland's Marsden Point, pictured on February 17. Photo / Michael Cunningham

“At Ruakākā Beach, where there was a really large foredune, it could take years to decades to build that back – and that’s contingent upon the sand that was carried away making its way back onto the beach face, and eventually back into the dunes.”

Unfortunately, the worsening impact of climate change – which could mean as much as 1.2m of sea level rise in many parts of our coast this century, on top of heavier storm surge - didn’t bode well for the long-term picture.

“This recovery comes against a backdrop of sea levels which are higher, and continuing to rise.”