The most powerful cyclone ever to slam into the South Pacific tourist destination of Tonga has destroyed villages, flattened trees and left at least one person dead, officials said Sunday.

Up to 70 percent of houses and buildings in the central Ha'apai islands group, which is home to about 8,000 people and bore the brunt of Cyclone Ian, were damaged or destroyed.

The Tongan government declared a state of emergency in the Ha'apai region after it was pounded by winds in excess of 200 kilometres per hour which whipped up mountainous seas around coastal villages.

Although initial reports when the cyclone hit on Saturday said there had only been minor damage, the full extent of the destruction began to emerge when communications were partially restored a day later.

"Seventy percent of houses (on Ha'apai) are damaged or blown away, and the rest of the 30 percent are affected by water," Tongan military commander Satisi Vunipola told reporters.

Residents on Ha'apai's main island of Lifuka were reported to have huddled in churches for shelter as houses were destroyed in the furious cyclone.

Ian Wilson, a New Zealand emergency management official, said Lifuka was in the direct path of the cyclone.

"Whatever was on the island has been damaged, whether it's buildings, crops, roading or infrastructure, it's all been damaged," said Wilson, warning that it remained difficult to get a detailed picture of the destruction.

"There is no communication. We did have a satellite phone but that also died. It is serious. The eye of the storm went right across the top of the island."

Ian was downgraded to a category four cyclone on Saturday morning, but increased in intensity later in the day to be restored to the most severe rating of category five as it hit Ha'apai, knocking out contact with outlying islands.

Ian is the first category five cyclone to belt into Tonga and Ha'apai governor Tu'i Ha'angana said he could see from one side of the island to the other -- "that's how devastated it is".

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 as communication remained sketchy.

He said staff in the region told him by satellite phone that Lifuka was devastated.

The Red Cross established a policy last year of maintaining containers of relief supplies on most islands, and Taumoefolau said they were able to provide immediate assistance.

Tupou Ahomee Faupula, from Tonga's cell phone provider Digicel, said his field officer in Ha'apai 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 has sent two patrol boats to Ha'apai, and the Matangi Tonga news website reported the government was considering a request for overseas aid.

New Zealand offered immediate assistance of US$41,500 and an Air Force Orion was sent Sunday to begin an immediate aerial surveillance of the devastated areas.

The flight was not expected to return to the capital Nuku'alofa until late at night.

"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 259 kph.

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