The tsunami that killed more than 200 people in the Samoan islands and Tonga in September was twice as tall as the buildings it hit, scientists say.
The wall of water towered 14m (46ft) and included as many as three major waves, researchers have found.
The waves were caused by a magnitude 8.0 earthquake under the sea.
New Zealand's National Institute of Water and Atmospheric Research has been studying the tsunami to help guard against future events.
A team of scientists under NIWA spent two weeks in the field and found the second of two to three significant waves was said by witnesses to be larger.
The delay between the earthquake and the arrival of the first wave was about 10 minutes in Samoa and 20 minutes in American Samoa, the NIWA scientists said.
It was also very clear that plants, trees, and mangroves reduced flow speeds and water depths over land - leading to greater chances of human survival and lower levels of building damage.
"The same thing will be true in New Zealand as in Samoa: solidly constructed buildings which are appropriately located will survive much better than flimsy buildings right on the beach," said Dr Stefan Reese of NIWA.
"It's also clear that practices such as flattening sand dunes or removing beach vegetation would increase the potential for tsunami damage."
The massive waves that struck Samoa, American Samoa and Tonga totally destroyed traditional wooden buildings, many of them single-storey, along the coast.
The scientists measured the watermarks on buildings and trees to help confirm the height of the waves, which surged as far as 700 metres (765 yards) inland in some areas.
They also found that the Samoan quake created a sea floor fault up to 300km (190 miles) long and 7m (23ft) deep.
NIWA's Dr Shona van Zijll de Jong said that the tsunami may have permanently changed residential patterns in Samoa.
"Many people are scared of the sea, and people are staying away from devastated villages," Dr van Zijll de Jong said.
"The sea has been a source of livelihood and identity for generations. The violence of the tsunami really shook them.
"Their sense of personal security and economic well-being is deeply shaken."
The 29 September tsunami killed 34 people in American Samoa, 183 in Samoa and nine in Tonga.