Tongan authorities fear thousands of families have "totally lost" their homes, and are aiming to start distributing aid from Wednesday afternoon.
The main island of Tongatapu, as well as 'Eua - nearby to the southeast - were badly damaged by Tropical Cyclone Gita overnight Monday. On Tuesday a New Zealand air force Orion carried out an aerial surveillance of the islands.
Graham Kenna from Tonga's National Emergency Management Office said on Wednesday morning people in lowlying areas and those closest to the coast seem to have been worst affected.
"They have lost the houses. We're concentrating on people who have totally lost their houses. That could be in the thousands of families." he said.
Emergency supplies had arrived from the New Zealand and Australian governments, and the aim was to start distributing those to the most needy from Wednesday afternoon. The supplies included shelter, and kitchen and hygiene kits.
On Tuesday efforts had concentrated on clearing roads and trying to get around and do assessments.
"We still can't do a total assessment of the island (Tongatapu). Some roads are impassable, and we just don't have enough people to do it," Kenna said.
Not everyone could get to work, and people had been told to prioritise the needs of their own families.
Electricity was out across Tongatapu, which also meant no power to pump water.
Hopefully power would be restored to the hospital on Wednesday, with water possibly by Thursday, Kenna said.
"With people with serious injuries we can't do CAT scans on them. That's a major issue. Hopefully that will be up by midday."
Three people are thought to have been seriously injured during the storm, while another 30 have lesser injuries. The storm may also have contributed to the death of a 72-year-old man who had a heart attack.
Hopefully the CBD of Nuku'alofa could have at least some power in two or three days, Kenna said.
"Tonga Power can handle the issues we have now but they're starting to run out of hardware, such as poles and wires."
Communication with the island of 'Eua was difficult, and aerial surveillance by the New Zealand Orion was being used to work out the extent of the damage there.
Acting Commander Joint Forces New Zealand Air Commodore Kevin McEvoy said indications from the Orion's surveillance were that much of the infrastructure was intact.
"There is some damage obviously, though most of the roads appear open and cleared of debris," he said.
"There is still some surface flooding around and ... there are some basic services significantly degraded."
The New Zealand Defence Force would be working with the Tongan authorities to determine where help was needed. For example, looking at whether a survey of the port was needed to see if anything was submerged.
"There are a number of containers strewn around the port area, and some damage to port facilities," McEvoy said.
"We have seen that there are powerlines down and no power operating and limited running water." A concern in any emergency was the risk of waterborne diseases.
"Really it's a case of getting onto the ground and seeing today, and in coming days, where the main effort needs to be." An air force Hercules that took 12 tonnes of emergency aid to Tonga on Tuesday also had a 10-member assessment team on board.
The Hercules and the Orion that did the aerial surveillance had returned to New Zealand on Tuesday night and were on standby if they were needed again, McEvoy said.
"At the moment there are no requests from the Tongan authorities that they need either further support or supplies or further surveillance flights.
"If they ask we have got those ready, and we can do those at relatively short notice."