Cyclone Ian, which packed winds of up to 180 mph, hit the islands Saturday, causing significant damage to infrastructure in Ha'apai islands, TVNZ reported.
The governor of Ha'apai said the islands of Ha'ano, Lifuka and Foa are all devastated. Authorities have been unable to reach 23 of the islands by telephone.
Relief efforts are already under way, with navy vessels bringing tarps, tents, and other emergency supplies to the Ha'apai islands.
International aid came from New Zealand Sunday morning.
"We will make $50,000 immediately available to respond to specific requests for assistance," said New Zealand Foreign Affairs Minister Murray McCully. "Our thoughts are with the people of Tonga as they begin to come to terms with the damage caused by this cyclone."