Rina Devina was getting ready to go to sleep with her husband and two of her children at home on the Indonesian island of Sumatra when she heard a thunderous noise and someone shouted "flash flood!"

Hours of heavy rain and cold lava from nearby volcano Mount Marapi inundated two districts just before midnight on Saturday, sweeping dozens of people to their deaths and damaging homes, roads and mosques. More than a dozen remain missing.

Residents of Tanah Datar and nearby Agam district who survived recounted their horror when the flash floods tore through, carrying their neighbours away and submerging houses and buildings.

"The rain was very heavy, I heard the thunder and the sound similar to boiling water. It was the sound of big rocks falling from Mount Marapi," Rina, a 43-year-old housewife in Agam, told AFP.

"My house was OK but my neighbor's house was flattened by big rocks. Three of my neighbours died — the mother, the father, and the child. Another neighbour, an 85-year-old, also died."

The mother-of-three said the electricity was knocked out, leaving her unable to see anything before she escaped to the office of the village head.

She fled into the heavy rain with only the clothes on her back and her family by her side.

"It was pitch black, so I used my cellphone as a flashlight. The road was muddy, so I chanted 'God, have mercy' over and over again," she said.

Rina said she would stay at the shelter until the authorities told her it was safe to go home.

In the flood-hit district of Tanah Datar, the flooding left roads caked in mud, trucks sticking out of a nearby river and mosques smashed by logs and metal sheets.

Some buildings were covered by mud halfway up their walls, with a new, elevated road surface being driven on by locals on motorbikes above the street that used to exist there.

'Terrifying'

The rains turned their neighbourhood into a sea of mud, interrupted only by rooftops, debris and palm trees.

An AFP journalist there said the powerful rains had been "terrifying", forcing residents to seek urgent shelter.

Rescue teams have deployed rubber boats in search of bodies or survivors.

Budi Rahmat, a 44-year-old farmer in Agam, remembered hearing a thundering noise and rocks rolling down the road.

"My house was vibrating. I took a peak outside and saw water flowing," the father-of-five told AFP.

"My house thankfully is OK. It's only flooded by water, not rocks."

He said they were eventually all able to evacuate to safety at a relative's house on higher ground, including his youngest child, a two-year-old. 

Residents such as Budi breathed a sigh of relief they and their families had survived, while rescuers carried on the search for the missing in the hope they would be found alive.

"I paced around the house, back and forth, mulling whether or not I should evacuate. My mind was in total chaos," Budi said.

"The only thing I could think about was that I had to save my wife and kids."