The death toll from devastating floods that have ravaged southern Brazil for days reached 100 on Wednesday, authorities said, as the search continued for dozens of people still missing.

Nearly 400 municipalities have been affected by the worst natural calamity ever to hit the state of Rio Grande do Sul, with hundreds of people injured and 160,000 forced from their homes.

Many have no access to drinking water or electricity – or even the means to call for help with telephone and internet services down in many places.

On Tuesday, state governor Eduardo Leite had warned the human toll was likely to rise as "the emergency is continuing to develop" in the state capital of Porto Alegre and other cities and towns.

Some 15,000 soldiers, firefighters, police and volunteers were at work across the state, many in boats, and even jet skis, to rescue those trapped and transport aid.

Authorities urged people not to return to affected areas due to possible landslide and health hazards.

Many people are loathe to leave their homes for the safety of shelters amid reports of abandoned properties being looted.

"Contaminated water can transmit diseases," civil defence spokeswoman Sabrina Ribas warned on Wednesday.

The National Confederation of Municipalities said nearly 100,000 homes had been damaged or destroyed by unprecedented rains and floods in the state, with losses estimated at about 4.6 billion reais (more than US$900 million.)

Porto Alegre is home to about 1.4 million people and the larger metropolitan area has more than double that number.

The state's Guaiba River, which runs through Porto Alegre, reached historic levels and five dams are at risk of rupturing. (AFP)