The cyclone destroyed homes, churches and even the nation's historic Parliament House.
Cyclone Gita cleared away from Tonga on Tuesday, still packing winds of around 210 kilometres per hour with significantly higher gusts.
Tens of thousands of people were left without power as the main island of Tongatapu experienced a complete blackout.
Fijian authorities were working to establish contact with some remote southern islands on Wednesday after powerful Cyclone Gita took down communications, though the Pacific Island nation's most populated centres avoided serious damage.
The roof of the Tongan Meteorological Building was ripped off, forcing updates to be deployed by radio.
"Tonga is a close neighbour of New Zealand and we stand ready to assist our neighbours in times like these", Air Commodore McEvoy said.
"I have been involved in disaster response for 30-plus years and it was the worst situation I've been in", he added.
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"We got teams going out today (yesterday) to all the affected areas to assess the damage, the needs and see whether we have the capacity to respond".
"The only reports received of damage to homes are from Ono-i-Lau".
The Labour Minister said she had a sleepless night worrying about the safety of her parents in Tonga as the cyclone hit. Gita's eye was located near 21.1 degrees south latitude and 179.1 degrees east longitude, about 190 nautical miles south-southeast of Suva, Fiji.
"As hard as that will be, that's what the Defence Force does in terms of contingency planning".
New Zealand has released NZ$750,000 in emergency relief funding for Tonga and has said that its defense force is standing by to provide assistance to the islands as soon as the Tonga government specify the assistance it requires.
"Initial assessments of the damage are still coming in".
Damages in Tonga after TC Gita.
"The other staff members could only make it the next day and when they came they all fell in".