Residents on low-lying islands said several homes were flattened and crops were wiped out when the cyclone pounded their villages.
Updated: Severe Tropical Cyclone Gita remains Category 5 status as of 4pm today over warm ocean waters 1800kms north of Auckland, New Zealand.
The full extent of damage is still being assessed.
He said many school buildings had had their roofs ripped off when Cyclone Gita struck.
There were no confirmed reports of deaths from the Category 4 storm, but there were a lot of injured people, some seriously, said Graham Kenna, an Australian government adviser at Tonga's National Emergency Management Office.
The main concern was for the islands of Ono-i-Lau and Vatoa, which were directly in the cyclone's path.
"At this stage, we have no reports of causalities and only minor injuries, so I think we can be grateful for that".
Power was slowly being restored after the late Friday storm.
Foreign Affairs Minister Winston Peters says New Zealand will contribute a further $1.5 million to help with the emergency response and early recovery efforts in the wake of Tropical Cyclone Gita.
The cyclone was intensifying and was forecast to hit some southern Fiji islands Tuesday night, though it's likely to miss Fiji's major population centers, including the capital, Suva.
All public schools are closed this week as downed trees, storm debris and utility wires are removed from campuses.
Tonga is a Polynesian kingdom that consists of more than 170 South Pacific islands, many uninhabited.
"We know many people will be desperately trying to get in touch with friends and family to make sure they're safe, especially over the coming days".
Desperate people boarded up their homes and stocked up on essential supplies while praying and hoping for the best before heading to the safety of emergency shelters.
Gita hit Samoa and American Samoa over the weekend, flooding the Samoan capital, Apia.