Disaster relief agencies are struggling to reach the remote tsunami-hit villages in Santa Cruz, Solomon Islands following Wednesdays powerful 8.0-magnitude quake and consistent strong aftershocks.
Aid agency, World Vision said the force of the surging water shunted some houses 10 metres back from the coast in the Ndende town of Venga and almost all the homes in Nela village were washed away.
"I'm currently walking through one community and I'm knee-deep in water," World Vision emergency coordinator Jeremiah Tabua said. "I can see a number of houses that have been swept away by the surge."
Unconfirmed reports said nine were killed and the national disaster management office said it had no clear picture of the scale of destruction on the island, more than 600 kilometres from the capital Honiara.
"Some of the remote communities we haven't heard back from yet, it's very difficult to get information," office spokesman Sipuru Rove said.
He said local officials were trying to check on the villages but the process could take days, as roads had been blocked by landslides and telecommunications was poor or non-existent.
World Vision said the latest reports from the aid group's staff on the ground suggested the scale of the disaster was bigger than previously thought and that up to 20 communities were believed to have been affected, rather than just five.
"All communities visited today have suffered significant damage. This brings the estimated number of people affected to 6,000 people approximately," it said in a statement, adding that food and water were running low in some areas.
The stricken island's airstrip was also closed because of debris on the runway, preventing planes carrying relief supplies from landing and thwarting plans to send reconnaissance flights over the disaster zone.
Officials said about 460 houses had been destroyed leaving some 3000 people homeless, with many villagers fleeing for higher ground.
Red Cross disaster manager Cameron Vudi said the death toll was likely to rise as reports filtered in from isolated communities.
"We're expecting changes. There are signs that there might be increases in the number of casualties," he said.
"There are still reports coming in. Most of the reports are confined to areas that are accessible by road but there are a lot more communities that have been damaged."
Mr Rove said the airstrip was expected to reopen today and the Solomons government had asked the Royal Australian Air Force to send a plane to survey damage.
The Patrol Boat carrying medical teams and emergency supplies such as tarpaulins, fresh water, food and clothing departed Honiara for Ndende yesterday and should expect to arrive over the weekend.
Both Australia and New Zealand said they were ready to provide assistance to their northern neighbour, with Wellington pledging an immediate $NZ200,000 ($163,000) for humanitarian supplies.
The US Geological Survey said Wednesday's quake struck in the middle of the day, beneath the sea about 76 kilometres west of Lata, Ndende's main town.
It was followed by dozens of strong aftershocks of up to 7.0 magnitude. The Hawaii-based Pacific Tsunami Warning Centre put several island nations on alert for two-and-a-half hours before declaring the threat had passed.