Strong aftershocks hampered efforts to reach tsunami-hit villages in the Solomon Islands Friday, as the death toll after an 8.0-magnitude earthquake rose to nine, with many more reported missing.

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."

A spokesman for Prime Minister Gordon Darcy Lilo's office, George Herming, said nine people were confirmed dead and an undetermined number missing, with the toll expected to rise as reports filter in from outlying areas.

He said the area was officially declared a disaster zone on Friday, as Australia announced it will assist relief efforts.

Australian Foreign Minister Bob Carr, who will visit the Solomons on Sunday, said the disaster "has been a devastating experience" for people in the low-lying islands.

In addition to the official toll, he said "many more are reported missing".

Carr announced Aus$300,000 ($309,000) in relief payments and said a Royal Australian Air Force Hercules had been sent to the Solomons to provide logistical support.

Rove said the plane would be deployed to make a reconnaissance flight over the island, giving officials their first comprehensive overview of the damage.

Attempts to fly aid to the remote area, more than 600 kilometres (370 miles) from the capital Honiara, have been hamstrung by damage to the airstrip at its main town Lata.

While debris stranded on the runway after it was inundated by surging waters had been cleared, a series of aftershocks rattling the island, including a 6.6-magnitude jolt Friday, meant pilots were wary about landing there.

Herming said a flight carrying Lilo and senior officials to the island had to turn back to Honiara Friday because of fears the 6.6 quake had caused fresh damage to the airstrip.

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 and were expected to arrive late Friday or early Saturday.

World Vision said food and water in the hillside camps was running low and sanitation would soon become an issue.

"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.

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.