The Solomon Islands Disaster Management Office says it is running out of food, water, clothing and toiletries, and rations are expected to run out today.
The death toll from flash flooding last week now stands at 23, with many more missing.
The office says more than 9,000 people have been left homeless, and are in 24 evacuation centres set up around the capital, Honiara, but this number continues to rise.
The office's director, Loti Yates, says sanitation is becoming a real concern and he fears an outbreak of disease including dysentry, malaria and dengue fever.
Honiara City and all of Guadalcanal province are still disaster areas and search and rescue missions are continuing.