More than two-thirds of the homes and buildings in some areas had been flattened, said Kalolaine Kavaefiafi of the child welfare charity Plan International.
Cyclone Ian hit Tonga on Saturday with gusts up to 178mph. The storm was later downgraded from category, five, the most destructive category of cyclone, to category four, with gusts of up to 155mph.
An aerial survey of the damage was carried out yesterday to assess the destruction and two navy patrol boats were sent to the disaster area, Tonga's director of emergencies Leveni Aho said.
He said of the damage: "It's pretty bad." A state of emergency remained in effect for two of Tonga's three island groups, Vava'u and Ha'apai.
Mr Aho said one person died on Lifuka island in the central Ha'apai group and many homes were damaged.