NUKU'ALOFA, Tonga (AFP) - A state of emergency was declared in parts of Tonga on Saturday as powerful Cyclone Ian slammed into the South Pacific island nation, bringing winds estimated at 105 knots (200 kilometres per hour).

But residents and officials said the maximum category five cyclone, which had been forecast to bring gusts of up to 155 knots, caused surprisingly little damage.

"There is a bit of damage at Ha'apai, some houses blown down at Foa but we still have telephone contacts with Ha'apai," Deputy Prime Minister Samiu Vaipulu told a media briefing.

Although Australia and New Zealand had aid services on standby, Mr Vaipulu said there was no reason to call for outside assistance.

"When it is over, we are going there (the outer islands). We are planning, if everything is over by tomorrow, we will assess it by tomorrow.

"New Zealand have got an aircraft on standby in Auckland already. Australia is also on standby to see what would be the needs from us but we have informed them so far there is not."

Ian was downgraded to category four late Saturday morning, although the Tonga Meteorological Service said it remained capable of "very destructive" winds of up to 145 knots.

"I think we were quite lucky," Mr Steve Campbell, an owner-skipper at the Ika Lahi game-fishing resort on the Vava'u island chain, told AFP.

"It was nothing like Waka back in 2001," he added, referring to a storm that caused more than US$50 million (S$63 million) in damage.

The cyclone ploughed overnight into the Vava'u area, one of Tonga's main tourist hotspots, and the director of emergencies, Mr Leveni Aho, said a state of emergency was declared when initial damage reports were received.

"It means we have the national resources ready to deploy in areas of need and authorities are given power to evacuate if needed," he told the Matangi Tonga website.

"But there doesn't appear to have been much damage in Vava'u at the moment."