Locals were left so spooked by weird tides at a Canterbury beach yesterday that GNS Science experts were asked to look into a potential tsunami.
But the reason for the dramatic fluctuations around Lyttleton weren't the result of an approaching monster wave, scientists say, but something else entirely.
Rosie Belton had planned to go for a swim at Governors Bay beach at high tide yesterday afternoon, but arrived to find that the tide was strangely low.
"It was a totally calm day – and one of many calm days in a row – yet the sea was full of ripped-up vegetation that was floating in it," she said.
"Then a woman who had been out canoeing commented on how strange the sea had been, and said she wasn't going to go out further than around the point."
When Belton walked into the surf for her swim, she fell into a hole that had been gouged in the sand.
"That was very unusual for that beach – and then we noticed there was plenty of gravel that had been brought in."
Stranger still was what she noticed next.
In the time she was in the water, the tide apparently climbed from three quarters' full, to full, and then back to low tide.
"I was at the beach for just over an hour and it just went up and down, just like a yo-yo."
That prompted a call to GNS Science experts, before the message was relayed to members of a special panel of tsunami experts.
Panel member Dr Jose Borrero, of Raglan-based eCoast Marine Consulting and Research, said as there were no reports of seismic activity, there were questions over what had caused the occurrence.
Borrero figured the shifting tides were the result of big swells on the east coast fuelling a seiche - a standing wave driven by pendulum-like movements of water levels that can occur within partially closed-in environments like a coastal inlet.
The swells themselves had been caused by a tropical system that flared up a few days ago to the east of Fiji, then dropped southward and re-intensified north east of East Cape.
He assumed the specific effect that Belton experienced was one of the well-known seiche modes of either Pegasus Bay or Lyttelton Harbour, which are driven by a combination of tides and waves from weather systems as they propagate over the continental shelf and into the bay.
But Canterbury-based tidal expert and consultant Derek Goring said the shifting waters were actually the work of what are called infra-gravity waves.
These were long waves which could take between 25 and 120 seconds to roll in, and were generated in the process of large swell waves breaking on adjacent beaches.
"Often they are correlated with the tide, being up to twice as high at high tide than at low tide," Goring said.
"They are difficult to detect, and usually they are so small they cannot be felt, and thus are of no consequence to people swimming or surfing.
"However, for ports they are problematic because they cause ships to surge at berth, breaking mooring lines and endangering people, other ships, and infrastructure."