Unconfirmed reports said at least two people had been killed by Cylcone Evan, which hit Samoa on Thursday.
In the capital, Apia, homes were flattened and the Vaisigano River burst its banks, washing away cars, trees and other debris.
But fears the cyclone might hit Samoa a second time have receded.
New Zealand Foreign Minister Murray McCully told reporters that "the best guess" was that the cyclone would not return.
Samoa's Disaster Management Office said power was off throughout the country.
"Tanugamanono power plant is completely destroyed and we might not have power for at least two weeks," it said.
The village of Lelata, just outside Apia, was also ravaged by the cyclone.
The village looked "like a tsunami has struck" it, local reporter Keni Lesa was quoted as saying by the Associated Press news agency.
New Zealand's high commissioner in Apia, Nick Hurley, said the cyclone was bigger than other tropical storms he had experienced in the region, and its strength was unexpected.
"The unpredictable nature of this one has made it quite different," he said.
"The forecast winds did not give any indication of how strong the impact was going to be."
The cyclone generated winds of up to 165km/h (100 mph) as it hit Samoa.
A powerful cyclone has caused widespread destruction on the Pacific island of Samoa, cutting power, uprooting trees and triggering floods.