The floods have submerged entire villages, snapped transport and communication links and raised fears of disease spreading in relief camps crowded with people forced from their homes.
Large parts of Andhra Pradesh and Karnataka states have been inundated.
Air force helicopters dropped food and drinking water packages to hundreds of villages that remained cut off after roads were submerged or washed away.
In Karnataka, the worst-hit of the two states, 168 people have died and more than 300,000 have been displaced, authorities said.
In neighbouring Andhra Pradesh, 37 people have died and around 450,000 have been displaced and were sheltering in 100 relief camps, said state chief minister K Rosaiah.
“We are rushing medical teams and equipment to the camp to ward off outbreaks of disease in the camps,” Rosaiah said.
Flooding worsened after authorities released water from rain-swollen reservoirs and dams in both states to prevent them from bursting their banks.
Army soldiers used boats to ferry villagers stranded on rooftops to government schools or relief camps on higher ground.
Doctors were distributing water-purifying tablets and providing medical services in the camps, but facilities were poor and crowded.
In places where villagers refused to leave their homes, drinking water was being supplied by tankers.
Just weeks ago, most parts of Andhra Pradesh and Karnataka were suffering from severe drought due to the failure of monsoon rains.