The confirmed death toll is two but pictures taken by New Zealand Defence Force (NZDF) reconnaissance aircraft showed "alarming" scenes of a village destroyed on Mango island and buildings missing on nearby Atata island, said Tonga's deputy head of mission in Australia, Curtis Tu'ihalangingie.
"People panic, people run and get injuries. Possibly there will be more deaths and we just pray that is not the case," Tu'ihalangingie told Reuters.
Tonga is expected to issue formal requests for aid soon but in the meantime New Zealand said two ships, HMNZS Wellington and HMNZS Aotearoa, had set off with water supplies, survey teams and a helicopter.
Tonga police told the New Zealand High Commission that the death toll from Saturday's eruption and tsunami was two but with communications in the South Pacific island nation cut, the true extent of casualties was not clear.
Australia's Minister for the Pacific Zed Seselja said Tongan officials were hoping to evacuate people from the isolated, low-lying Ha'apai islands group and other outer islands where conditions were "very tough, we understand, with many houses being destroyed in the tsunami".
The United Nations had earlier reported a distress signal was detected in Ha'apai, where Mango is located. The Tongan navy reported the area was hit by waves estimated to be 5-10 metres high, said the UN Office for the Coordination of Humanitarian Affairs (OCHA).
Atata and Mango are between about 50 and 70km from the Hunga Tonga-Hunga Ha'apai volcano, which sent tsunami waves across the Pacific Ocean when it erupted with a blast heard 2300km away in New Zealand.
Atata has a population of about 100 people and Mango about 50.
"It is very alarming to see the wave possibly went through Atata from one end to the other," said Tu'ihalangingie.
Briton Angela Glover, 50, was killed in the tsunami as she tried to save the dogs she looked after at a shelter, her brother said.
A thick layer of ash blankets the islands, the aerial images provided to Tonga by New Zealand and Australia showed.
The archipelago's main airport, Fua'amotu International Airport, was not damaged but heavy ashfall is preventing full operations, hampering international relief efforts.
People on the west coast of the main island of Tongatapu had been evacuated because of "significant damage", OCHA said, while government ministers had broadcast warnings against price gouging amid worries about shortages of supplies.
Australian Foreign Minister Marise Payne said C-130 aircraft from Australia could deliver humanitarian assistance including water purification supplies, while the HMAS Adelaide, which would take five days to get to Tonga, was ready to take engineering and medical teams and helicopter support.
Japan said it hoped to help quickly and would consult others in the region.
"The impact not just of the inundation, but of the extraordinary volume of ash which is covering everything, plus the communications issues, of course, makes this very difficult," Payne said.
Scientists say the eruption could deliver long lasting damage to coral reefs, coastlines and fisheries in the region, as well as causing acid rain
The archipelago has remained largely cut off since the eruption which severed its main undersea communications cable
Subcom, a US based private company contracted to repair subsea cables in the Asia-Pacific, said it was working with Tonga Cable Ltd to repair the cable that runs from Tonga to Fiji.
Samiuela Fonua, the chair of Tonga Cable, said there were two cuts in the undersea cable that would not be fixed until volcanic activity ceased, allowing repair crews access.
Tonga is a kingdom of 176 islands, of which 36 are inhabited, with a population of 104,494 people.
