Officials said six people were killed, and 11 others were missing in the Guangxi region by Tuesday evening.
The severe weather forced 130,000 people to evacuate, according to the regional propaganda office.
Some residents were still waiting to be rescued on Wednesday.
Lu Xiaofei, who works in Shenzhen, a tech hub near Hong Kong, said her brother's family was stuck at his house in Lu village in Qintang district.
His brother was with his wife, his nine-month-old baby, their parents and grandfather.
"The water in the house is over one person's height. They have to all move to the second floor. The power has been out since yesterday morning, and now they don't have running water, either," Lu told The Associated Press in a phone interview.
Lu said her brother told her that the water level rose on Tuesday night and their situation was so dire that drinkable water would be running out soon, while local authorities had not reached out to them.
Many villagers nearby were facing the same challenges, she said.
Others called for help on social media, showing videos of their inundated surroundings following the flooding and highlighting their lack of resources.
Local media Litchi News reported that snakes from breeding facilities were washed away and were at large in Hengzhou city, with some attempting to enter houses.
It quoted a villager saying that more than a dozen of people were bitten.
The national meteorological centre said heavy rain has been battering central-eastern and southern parts of Guangxi since last Saturday, with cumulative rainfall amounts reaching 100 millimetres to 400mm in some areas.
Some hard-hit areas recorded more than 900mm, it said.
The weather agency said heavy rain would continue to affect the areas on Wednesday.
Some train services were suspended in the region for safety reasons.
While Guangxi was grappling with floods, Super Typhoon Bavi is expected to hit parts of southeastern China at the weekend.
Elsewhere in Asia, severe weather also caused deaths.
Landslides triggered by heavy monsoon rains in southeastern Bangladesh killed several Rohingya refugees, including five children.
Heavy monsoon rains battered parts of neighbouring India, leaving over a dozen of people dead in the past few days.
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