Strong aftershocks hampered efforts to send aid to tsunami-affected villages in the Solomon Islands Friday, as the death toll following a powerful 8.0-magnitude earthquake rose to nine.

Officials said damage on Ndende island in the eastern Solomons was much worse than first thought, with up to 20 villages swamped in a tsunami generated by Wednesday's quake and 6,000 people homeless, doubling previous estimates.

"At first we thought it was going to be quite small but now it looks like it's going to be very big and communities will not be able to handle it themselves," national disaster management office spokesman Sipuru Rove told AFP.

"This is where we might require external assistance."

The remoteness of the disaster zone, more than 600 kilometres (370 miles) from the capital Honiara, has hamstrung relief efforts, with plans to fly in aid thwarted by damage to the island's airstrip in the main town Lata.

Rove said debris left on the runway after the quake had been cleared but the constant aftershocks rattling the island, including a 6.6-magnitude jolt early Friday, meant pilots were wary about landing there.

He said a flight carrying Prime Minister Gordon Lilo and senior officials to the island was believed to have turned back to Honiara early Friday because of fears the 6.6 quake had caused fresh damage to the airstrip.

In addition, Rove said the aftershocks were frustrating attempts by villagers to salvage supplies from their devastated homes to equip the makeshift camps where they are sheltering in the rugged interior.

"They will go down (to the villages) but when there's another aftershock they run back to the hills because they're scared of another tsunami," he said.

With the airfield still out of action, two boats set sail from Honiara late Thursday loaded with food, water and medical supplies. Rove said they were expected to arrive late Friday or early Saturday.

World Vision, which had a 20-strong team working on development projects on the island before the tsunami, said food and water in the hillside camps was running low and sanitation would become an issue in coming days.

"Destruction has been widespread in and around Lata," it said. "Coastal wells have been covered by debris or contaminated, water tanks and toilets have been destroyed and coastal areas are littered with dead fish and poultry.

"For many families, their livelihoods also disappeared in the water."

Both Australia and New Zealand said they are ready to provide assistance to their northern neighbour and Rove said an Australian air force Hercules would set off from Honiara Friday to make a reconnaissance flight over the island.

The US Geological Survey said Wednesday's quake struck in the middle of the day, beneath the sea about 76 kilometres west of Lata.

The Solomons are part of the "Ring of Fire", a zone of tectonic activity around the Pacific that is subject to frequent earthquakes and volcanic eruptions.

In 2007 a tsunami following an 8.0-magnitude earthquake killed at least 52 people in the Solomons and left thousands homeless. The quake lifted an entire island and pushed out its shoreline by dozens of metres.