The authorities opened sluice gates of four major hydroelectricity reservoirs in central Sri Lanka late on Saturday to prevent damage to dams.
Seven people were killed in Sri Lanka last month when a tropical cyclone brushed the island's eastern coast.
The navy and airforce are searching for 36 fishermen who have been missing at sea since the monsoon hit early on Saturday, Sarath Kumara Gunaratne, deputy minister of disaster management, told reporters.
The bodies of 18 fishermen have so far been found, while the body of another person was discovered on land, the minister said.
"A joint rescue operation is underway," the minister said.
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"The navy and the airforce have deployed several craft to look for survivors. At least 21 injured fishermen have been rescued so far and admitted to hospital," he said.