In Indonesia, the meteorological agency warned the three hardest-hit provinces on the island of Sumatra will see "moderate to heavy" rain between Thursday and Friday.

The downpour started overnight, but so far has not reached near the intensity that led to destructive flash-flooding and landslides last week.

The toll on Thursday stood at 776, revised down slightly from a day earlier as information arrives from remote, inaccessible areas.

More than 560 people remain missing, with patchy communications and electricity making it hard to confirm whereabouts.

At a shelter in Pandan in North Sumatra, 54-year-old Sabandi told AFP she was still traumatised by the floods that swept feet of mud into her home last week.

"We feel scared," she said of the forecast.

"We are afraid that if it rains suddenly, the flood will come again."

She waited out the floods on her roof for two days, stranded without food or water, before she could evacuate.

"My house was filled with mud. The mud was so high that we couldn't enter the house," she added.

While across Asia seasonal monsoons bring rainfall that farmers depend on, climate change is making the phenomenon more erratic, unpredictable and deadly across the region.

Two separate weather systems dumped massive rainfall on all of Sri Lanka, Sumatra, parts of southern Thailand and northern Malaysia last week.

The scale of the disaster has made relief efforts challenging.

In Indonesia's Banda Aceh, an AFP reporter said the line for fuel at one gas station extended four kilometres.

Elsewhere, survivors reported food shortages, price gouging and looting.

- Monsoon rains in Sri Lanka -

In Sri Lanka, forecasters said the northeast monsoon was due to arrive from Thursday afternoon.

Landslide alerts were renewed for some of the worst-hit areas of the central region, and residents were advised not to return home as the already saturated slopes could collapse under more rainfall.

The main highway from Colombo to Kandy, a distance of 115 kilometres (71 miles), was reopened for 15 hours a day, as workers cleared mounds of soil and boulders.

On an alternative route between the regions, an AFP reporter saw traffic moving at a crawl as vehicles navigated badly damaged tarmac.

Jagged tears on the mountainside revealed fresh expanses of soil, stark against lush greenery.

At least 479 people have been killed in Sri Lanka, and hundreds remain missing, with the president appealing for international support.

Authorities estimate they will need up to $7 billion to rebuild homes, industries and roads, a tough ask for a country still emerging from its worst-ever economic crisis three years ago.

Outside the capital Colombo on Wednesday, some residents were returning to homes still filled with floodwaters a foot high.

Soma Wanniarachchi, 69, had stayed behind as long as she could, "but when the water level reached about eight feet (2.5 metres), I decided to leave," she told AFP.

Back in her village of Kotuwila, near Colombo, she was shocked to see the damage to her catering equipment rental business.

"My stainless steel utensils are now probably in the Indian Ocean," she said.

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