Tsunami debris along the seaward bank of the railway line at the south end of Oaro Bay.

The tsunami that followed the Kaikoura Earthquake reached a peak height of about 7 metres, researchers have confirmed.

"It was much higher than any of us imagined," Niwa environmental scientist Darren-Ngaru King said.

He, with fellow members of Niwa's post-tsunami survey team Dr Emily Lane and Jochen Bind, found the tsunami was highest at Goose Bay, and only slightly less high at Oaro Bay.

NIWA Tsunami damage to the Little Pigeon Bay cottage.

Oaro – 22 kilometres by road south of Kaikoura – is the first place on the trip north from Christchurch where State Highway 1 runs alongside the ocean. Goose Bay is a few kilometres closer to Kaikoura.

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The team surveyed the areas on December 9 – three-and-a-half weeks after the magnitude-7.8 Kaikoura earthquake, which struck just after midnight on November 14 wreaking massive landscape changes to North Canterbury and Marlborough.

Findings were based on marine debris, such as logs, kelp, fish, crayfish and paua found well above where they would normally be expected.

The tsunami was found to be highest at Goose Bay, with data indicating a maximum runup height above tide level at the time of the tsunami of 6.9m + or - 0.3m. At Oaro the height was 5.3m + or - 0.3m.

Although the survey was more than three weeks after the earthquake there was still evidence "all over the place" to show the height of the tsunami, King said. "It was really quite remarkable."

For example, yellow-eye mullet had been thrown up onto the railway line alongside the beach and some had washed over the railway line onto the road behind.

Crabs were found a couple of hundred metres up some local streams, and bull kelp was found in the upper reaches of a freshwater stream.

That evidence was supported by local residents recounting what they saw on the morning after the earthquake.

It may have been fortunate the earthquake happened when it did, rather than during the day when people may have been walking along the beach, King said.

The survey team also found marine and freshwater flora and fauna scattered across the Oaro River flood plain – upstream of the railway bridge at the southern end of Oaro – extending 250m from the high tide mark on the day of the survey.

That inundation distance was likely an underestimate of the maximum distance inland the tsunami wave or waves penetrated, King said.

At Goose Bay, traces of bull kelp were scattered across the banks of the Otemakura Stream and necklace varieties of seaweed were seen draped from tree branches overhanging the stream as far as 170m from the high tide mark. That was also likely to be an underestimate of the maximum inundation distance.

Lane, a hydrodynamics scientist at Niwa, said research was still being done to understand why the tsunami was highest at Oaro and Goose Bay.

A main cause of the tsunami was thought to be rupture of the Hundalee Fault that extended offshore just north of Oaro. There was uplift where the fault ruptured on land of about 2m, and that would have continued offshore.

Given how close the Hundalee Fault was to the area, the first wave would have been almost immediate. It was not known whether that would have been the biggest wave, Lane said.

Other faults in the area that ruptured could have contributed to the tsunami, while another factor was that run-up heights tended to be higher at steep beaches, such as those at Oaro and Goose Bay.

Underwater landslides were also being considered as a possible cause of the tsunami, but from what was known so far they appeared to have been smaller than would have been expected for the size of tsunami.

GNS Science senior geophysicist Dr William Power said the heights at Oaro and Goose Bay were exceptional. Many other measurements were in the 3-4m range. "That's still significant," he said.

"This is one of the most significant tsunami for probably half a century. It's quite a big deal."

Luckily the tsunami happened between low and mid-tide. That combined with the uplift of the land by 1-2m limited the damage caused.

"It's the raising up of the seabed offshore that causes the tsunami, but the land around the coast also raised up, in many places 1m, sometimes 2m," Power said. "That helped to, in a way, protect the coast from more damage."

Immediately after the earthquake the tide level at the Kaikoura tide gauge started dropping. Over 25 minutes it dropped about 2.5m, a classic warning sign of a tsunami.

During the next 15 minutes the water level rose from its lowest level by about 4m. That was followed by a series of waves over several hours, Power said.

The water level at the Kaikoura gauge rose to 2.5m higher than it would have been. That was made up of a 1.5m rise measured on the gauge, and a rise of about 1m of the gauge itself, as the seabed and surrounding land rose by that amount.

Some other tide gauges that recorded the tsunami were in Wellington Harbour, Castlepoint, Christchurch and the Chatham Islands.

Tsunami height could be very localised and very influenced by the shape of the coastline.

"You probably have a little feature, maybe a creek or something, where the water gets pushed in and gets squeezed up," Power said.

At the V-shaped Little Pigeon Bay on the north side of Banks Peninsula, where a vacant cottage was inundated, the runup height was about 3m, with the water probably about 1m deep inside the cottage.

"An adult probably would have survived, however an infant or person of limited mobility would probably have been in great danger," Power said.

There was 140-150m of inundation into the back of the bay. The V-shape was probably a factor in the water being squeezed into the bay.

More details of the tsunami are expected to become available soon in a jointly authored study by GNS, NIWA and UNSW Australia.