The full extent of the destruction began to emerge after communications were partially restored a day after Tonga's first category five cyclone struck early yesterday morning.
Initial reports yesterday said the cyclone had left minor damage.
Ian was downgraded to a category four cyclone late yesterday morning, but increased in intensity later in the day and was restored to the most severe rating of category five.
By today there were reports of houses destroyed and trees flattened across the island chain, which is home to about 8,000 people and popular with tourists.
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He said staff in the region told him by satellite phone the main island of Lifuka was devastated.
Tupou Ahomee Faupula from Tonga's cell phone provider Digicel said his field officer in Ha'apai, Uaisele Fonokalafi, reported widespread devastation.
"He told us that this was the worst ever damage from a cyclone. Most houses are flattened, roofs are off, trees and power lines are down."
The Tonga navy was sending two patrol boats to Ha'apai and the Matangi Tonga news website reported the government was considering overseas aid.
"Ian is still a category five this morning," Aisea Akau'ola, at the Fua'amotu Tropical Cyclone Warning Centre said today.