Raging floods and mudslides this past week have killed at least 55 people in southern Brazil and forced nearly 70,000 to flee their homes, the country's civil defence agency said.

Authorities said least 74 people were injured and another 67 were missing from the catastrophic flooding.

Fast-rising water levels in the state of Rio Grande do Sul were straining dams and threatening the economically important city of Porto Alegre, home to some 1.4 million residents.

The Guaiba River, which flows through the city, was at a historic high of 5.04 metres, well above the 4.76 metres that had stood as a record since the devastating floods of 1941.

Authorities scrambled to evacuate swamped neighbourhoods as rescue workers used four-wheel-drive vehicles -- and even jet skis -- to move through waist-deep water in search of the stranded.

Civil defence officials also said more than a million people lacked access to potable water amid the flooding, describing damage as incalculable. (AFP)