The director of the national disaster office, Loti Yates, said much of the plains are under water after days of torrential rain, and several villages have lost their food source.
Mr Yates said the plains also supply much of the food for the densely-populated capital, Honiara.
"These are the communities that are actually feeding Honiara and the markets, and so that's going to impact on their livelihoods as well. So it's going to be a huge challenge for us."
Loti Yates said assessments are still being carried out, and the full extent of the damage should emerge today or tomorrow.