At least one person was killed when powerful Cyclone Ian ploughed into Tonga's northern Ha'apai islands, causing extensive damage and destroying houses, reports said Sunday.

The full extent of the destruction began to emerge when communications were partially restored a day after the South Pacific kingdom's first category five cyclone struck early Saturday morning.

Initial reports on Saturday said the cyclone had left minor damage.

Ian was downgraded to a category four cyclone late Saturday morning, but increased in intensity later in the day and was restored to the most severe rating of category five.

By Sunday there were reports of houses destroyed and trees flattened across the island chain, which is home to about 8,000 people and is popular with tourists.

The head of the Tonga Red Cross, Sione Taumoefolau, said he had been informed of one death in Ha'apai but did not have further details.

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.

New Zealand has announced immediate assistance of NZ$50,000 ($41,500) and an Air Force Orion to assist with aerial surveillance of the devastated areas.

"Our thoughts are with the people of Tonga as they begin to come to terms with the damage caused by this cyclone," said Foreign Minister Murray McCully.

"Further support will be considered as the full extent of the damage becomes clear and the government of Tonga determines its priority response areas."

The Fua'amotu Tropical Cyclone Warning Centre downgraded Ian again to category four Sunday, with wind gusts of up to 140 knots (161 miles per hour, 259 kilometres per hour).

The storm was expected to continue weakening as it moved south over open waters, away from the island nation, according to meteorologists.