Five people died in Qingyuan City, Guangdong province, state-run Xinhua news agency reported.
Qingxin District in Qingyuan has been worst hit by persistent heavy rain, which has caused four small- and medium-sized reservoirs to overflow.
Rain has flooded nearly 483 villages and 12,300 homes, toppled 45 houses and trapped 454 residents, local officials said.
In the district's Shitan Town, although floodwater has dispersed, more than 20,000 residents lack drinking water as a reservoir has been polluted, Liu Minyi, an official with the township government said.
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Sixteen highways were blocked by the flooding in Guangdong.
In Guangxi, three people died and nearly 280,000 people were affected by floods.
Some 15,600 people were relocated and another 8,100 are in need of daily necessities.
About 22,880 hectares of crops were affected and 696 houses collapsed, which caused direct economic losses of about 200 million yuan (USD 32.4 million).