The latest deaths brought to 40 the toll in the impoverished region that has been lashed by unusually heavy rains since the weekend.
All but one of the people who died on Thursday were in the city of Batman, most of them swept away by floodwaters when the Iluh river overflowed its banks and inundated several shanty towns, Batman province's Governor Haluk Imga said.
Others died when their houses collapsed.
Ten of the dead were children, including four-year-old twin sisters and four other siblings aged eight months to five years, officials said.
The bodies of two boys swept away by floodwaters from the neighbouring province of Diyarbakir were found on the banks of the Tigris river near the ancient town of Hasankeyf, Imga told Anatolia news agency.
A 40-year-old villager died in Sirnak province, which borders Iraq and Iran, as he tried to save his livestock from floodwaters, Anatolia reported.
Six people were missing across the south-east and one in Mersin province, further west on the Mediterranean coast, and 16 people were injured, officials said.
In Batman, military units aboard dinghies rescued dozens of people stranded in their homes as at least five neighbourhoods, mainly near the Iluh river, were completely flooded.
Hundreds of people who lost their homes sought shelter with relatives or in the city's sports arena.
"By the time I went out to see what was happening, the waters were neck-high," Batman resident Ozcan Arslan told NTV television as he told of the disaster that struck overnight.
"We were at home when the waters burst the door open," another resident, Murat Aslan, said. "I grabbed the children and climbed to the roof."
The head of the chamber of construction engineers in Diyarbakir, where 18 people died on Wednesday, blamed the high toll on faulty building practices in the region, notably on settlements built in river basins.
"Nowhere in the world, is construction authorised in river basins," Tansel Unal said. "Negligence and badly planned urbanisation are the real cause of high death tolls, be they due to floods or earthquakes."
Imga, the Batman governor, agreed.
"The most important reason for the deadly disaster that has befallen us is settlement in river basins," he said, blaming local administrations for licensing construction in risky areas.
The 22-year conflict between the Turkish army and separatist Kurdish rebels in the south-east has forced tens of thousands of people to flee rural areas and settle in shoddily built suburban shanty towns.
In addition, many houses in rural areas are built of mud brick and are highly vulnerable to floods and earthquakes.
Traffic was paralysed in many parts of the south-east, Turkey's poorest region, as floods destroyed bridges and roads, caused power and telephone failures and killed hundreds of head of livestock.
A major plant in Batman of the national oil refiner TUPRAS stopped operations.
In the town of Cinar near Diyarbakir, several dozen people gathered outside the district governor's office to protest against what they called insufficient government efforts to help the flood victims.
Several men hurled stones at the building, but the protest ended shortly after the district governor promised the demonstrators that their problems would be solved.
Weather services, meanwhile, warned that more rains were expected to hit the region at the weekend.

Copyright AFP (Agence France-Presse), 2006