Tom Allard, Indonesia – At least 50 Indonesians are believed to have perished and hundreds of homes destroyed after a dam broke south of Jakarta, sending huge volumes of water rushing through a shantytown while its residents slept.

The disaster happened about 4am, according to witnesses, and followed days of torrential rain in Tangerang, on Jakarta's southern outskirts.

"I heard heavy rain. Then it sounded like a wave," Yunita, a distraught resident, told Metro TV. "I don't know where my child is. My sister is dead and I don't know where my family is."

Yunita said her house was washed away moments after she heard the dam break and she became submerged in water.

Most of the victims were from a kampung, or working class neighbourhood, set amid running streams and creeks. Most of the residents had built their ramshackle homes from waste timber and corrugated iron.

The ageing dam was protecting the residents from nearby Gintung Lake and, according to Indonesia's Vice-President, Jusuf Kalla, was in poor condition. Jakarta is coming to the end of its five-month rainy season, so the dam was full to overflowing.

The death toll was so high because most of the victims were asleep and had no chance to escape. The wave crashed into about 400 homes in the industrial area, the head of the Health Ministry Crisis Centre, Rustam Pakaya, said.

Floodwaters were up to 2.5 metres deep is some areas, police and witnesses said.

"A flash flood came suddenly and was horrifying," said Seto Mulyadi, whose car was washed nearly 100 metres from his driveway into a public park.

Mr Mulyadi said he heard a siren sound at the dam before the water smashed out all the windows and doors and inundated his home.

Mr Pakaya said 32 bodies had been recovered by rescue teams so far, but he expected the death toll to climb.

In some places water levels reached rooftops and television footage showed several bodies floating amid chairs, clothing and other debris. Telephone lines were toppled and cars swept away, some hundreds of metres.

The 10-metre-high dam, which was holding back about 2 million cubic metres of water in a lake fed by the Pesanggrahan river, was built about a century ago, while Indonesia was still under Dutch colonial rule, according to a city official, Dadang Arkuni.

Authorities were trying to determine the cause of the accident. Some said heavy rains caused the dam to first overflow and then, because the foundation was not made of concrete, to burst.