Officials described Severe Cyclone Gita as the most powerful storm to hit the capital, Nuku’alofa, packing winds exceeding 230 kmh (142 mph) and dumping 160mm (9.0 inches) of rain.
“It was a particularly bad night,” Graham Kenna, from the National Emergency Office, told Radio New Zealand.
“I’ve been involved in disaster response for 30-plus years and it was the worst situation I’ve been in.”
The howling winds peaked in the early hours of Tuesday and McKenna said teams were heading out at first light to assess the damage.
But he said it was already clear there was widespread destruction in Nuku’alofa.
“Someone’s roof is in my front garden... some of the old landmark buildings that are 100 years old have been severely damaged by debris,” he said.
McKenna said the military was clearing roadways and had rescued some residents, including a pregnant woman who went into labour and was taken to hospital.
He was unaware of any fatalities, although unconfirmed media reports said two people had died.
The storm cut power to the main island Tongatapu on Monday night, leaving many of its 75,000 residents in darkness as the winds battered them and debris crashed against buildings.
But phone networks continued to operate and some of those affected shared their experiences online.
“Hotel is shaking and can hear screech of metal flying past outside.. rain water coming in sideways to second floor room. Heart with those not in solid structure this is bad gita #tonga,” TVNZ’s Pacific correspondent Barbara Dreaver tweeted.
Lord Fusitu’a, a hereditary peer in Tonga’s parliament, wrote: “I can literally feel my room shaking to the foundations - it’ll be a long night.”
Yet as the cyclone moved out to sea early Tuesday, he said Nuku’alofa had experienced “a dose of miracle”.
“Astoundingly #TCGita seems to have subsided & we’ve avoided the worst of it,” he wrote on Instagram, saying the focus would now move to rebuilding.
The storm had already created havoc when it slammed into Samoa on the weekend and authorities in Tonga declared a state of emergency before it hit Nuku’alofa.
However, Gita did not intensify into a maximum Category Five as feared and passed just south of the capital rather than making a direct hit.
McKenna said the impact of storm surges on the flat terrain of Tongatapu was also less than expected.
“It was very fortunate that the worst of the cyclone came at a dead low tide, so it sort of worked in our favour,” he said.
Cyclones are common in the Pacific at this time of year and Category Five systems have proved catastrophic when they make landfall.
Cyclone Winston killed 44 people in Fiji in 2016, and Cyclone Pam claimed 11 lives and damaged 65,000 homes in Vanuatu in 2015.