Monsoon flooding has forced more than 30,000 people in eastern Sri Lanka from their homes, with many taking refuge in schools, mosques and huts, officials have said.

"Due to the activation of the northeastern monsoon, 31,042 people have been displaced," H Dharmapriya, assistant director at the National Disaster Management Centre, said.

"Of that, 10,869 people are in 20 IDP [internally displaced people] camps," Dharmapriya said.

"The water levels are receding and there are no heavy rains as such at the moment," he added, saying one third of the displaced were in temporary shelters while the rest were housed with relatives.

Flooding hit the eastern district of Ampara, a largely flat agricultural area with a coastline that was hammered by the 2004 tsunami and where infrastructure has long been neglected because of a protracted war between the state and Tamil Tigers, after heavy rains began on Monday.

Mass displacement of people following seasonal floods are common in Sri Lanka.

The meteorology department forecast continued rainfall in coming days, but said it would not be heavy.

Flooding and drought are cyclical in Sri Lanka, where a southern monsoon batters the island between May and September, and a northeastern monsoon runs from December to February.

In December, 20,000 people were displaced in Ampara as a depression over the Bay of Bengal compounded monsoon rains.

The same month, 175,000 people took refuge in welfare centres and temples in the eastern and central parts of the country following flash floods.