Authorities reported up to 132 millimetres of rainfall in southern Sri Lanka over a 15-hour period ending Thursday night.

But while the deluge was intense, they said the large-scale flooding seen since last week had begun to subside.

The Disaster Management Centre (DMC) said 486 people had been confirmed killed and another 341 were still unaccounted for after Cyclone Ditwah left the island on Saturday.

The number of people in state-run refugee camps has dropped to 170,000 from a peak of 225,000 as floodwaters receded in and around the capital Colombo.

Record rainfall triggered floods and deadly landslides, with President Anura Kumara Dissanayake saying it was the most challenging natural disaster to hit the island in its history.

Residents evacuated from the landslide-prone central hills have been told not to return immediately to their homes, even if they were unaffected by the slides, as the mountainsides remained unstable.

In the central town of Gampola, residents worked to clear the mud and water damage.

"We are getting volunteers from other areas to help with this clean-up," Muslim cleric Faleeldeen Qadiri told AFP at the Gate Jumma Mosque.

"We have calculated that it takes 10 men a whole day to clean one house," said a volunteer, who gave his name as Rinas. "No one can do this without help."

The top official in charge of the recovery, Prabath Chandrakeerthi, Commissioner-General of Essential Services, said authorities were paying 25,000 rupees ($83) to clean a home, with costs of reconstruction as much as $6-7 billion.

A further 2.5 million rupees ($8,300) is being paid to begin rebuilding destroyed homes. More than 50,000 houses had been damaged as of Friday morning, officials said.

Chandrakeerthi's office said nearly three-quarters of the electricity supply across the country had been restored, but some parts of the worst-affected Central Province were still without power and telephones.

President Dissanayake declared a state of emergency on Saturday and has vowed to rebuild with international support.