Most of the Solomon Islands' main province of Guadalcanal has been declared a disaster area after widespread floods left nine people dead and more than 1,000 families homeless, officials said Friday.

There are fears the death toll could rise as several people are missing, including a Belgian national who went trekking in the mountains a day before the torrential rains began.

Homes and crops have been destroyed and New Zealand Foreign Minister Murray McCully, who has maintained contact with Solomons authorities, said more rain was expected in the next few days.

"The Solomon Islands government has declared a state of emergency," McCully said in announcing 100,000 NZ dollars (50,330 US) in relief aid.

Australia has also given nearly 100,000 Australian dollars (62,300 US) for disaster relief and France has mobilised a patrol boat to ferry humanitarian supplies to isolated areas.

"The official death toll currently stands at nine, and over 1,000 families from 270 villages have been forced to leave their homes," McCully said.

Solomons officials put the number of homeless at more than 1,800 families. The weather office forecast continued heavy rain in the coming days and issued a cyclone warning.

McCully said the funding would help the Guadalcanal provincial government establish a disaster centre and charter ships to help with the relief effort where bridges have been washed away.

A large area of West Guadalcanal has been isolated from the Solomons capital Honiara, the Guadalcanal Disaster Office said.

"Road transportation is totally cut off and it is a bit difficult for us to get relief supplies and medicines across to the flood-stricken victims," the provincial disaster coordinator Herrick Savusi told the Solomon Star.

He said stranded villagers were in desperate need of food, water and other supplies.

The Australian-led regional assistance mission in the Solomon Islands was also involved in the relief effort, helping with transport, damage assessment and medical evacuations, its special coordinator Graeme Wilson said.

Villagers in the worst affected areas blamed logging in the mountains for the tragedy.

"There's no doubt logging was a major contributing factor to this," Abraham Bara told the Solomon Star.

He said rain water flowed freely into the river, washing away topsoil and vegetation, because there were so few big trees left.