Songkhla's disaster prevention and mitigation office said eight of 16 districts _ Muang, Chana, Singha Nakhon, Ranot, Sathing Phra, Khuan Niang, Krasae Sin and Rattaphum _ were put on the disaster list as floodwater wrecked homes, cars and fields.
Senior official Amnart Polamat said the most critical areas are in Ranot, Sathing Phra and Chana districts.
Flat-bottomed boats have been sent to the areas to pick up stranded residents in flooded villages.
In Prachuap Khiri Khan, flash floods were reported in Bang Saphan Noi district yesterday.
Run-off from forests caused by the previous night's downpour inundated the municipality and raised the flood level to more than a metre.
Backhoes were used to dredge a leeway to help drain the excess water into the sea, according to the district office.
Bang Saphan Noi assistant district chief Thawil Chanthaworanurak said residents along the Bang Saphan canal and those in low-lying areas were issued with flood warnings.
Also hard-hit was Tha Chana district in Surat Thani, where floodwater continued to rise in the downtown area yesterday. Water was measured at 50-70cm.
The worst of the flooding was expected yesterday as more water was flowing downstream from the nearby tambons of Khlong Pha and Prasong.
About 2,000 residents in seven villages in tambon Khlong Pha were cut off after two main roads became impassable.
Tha Chana district chief Chumpol Suksai ordered the evacuation of the municipality as the main Khlong Pha reservoir began to overflow, sending down a rapid surge of floodwater.
In Phatthalung, floods damaged homes and farmland in Pa Bon, Pak Phayun, Bang Kaew, Khao Chaison, Khuan Khanun and Muang districts.
At least four tambons in Muang district and three in Khuan Khanun district were under as much as 1.5m of water, affecting more than 50,000 residents.
Phatthalung governor Seri Srihatrai said damage to properties was being surveyed. Food and necessities were being distributed to flood victims.
He said if the floods worsen, he will declare all 11 districts disaster zones.
In Trang, three people were killed as persistent wet weather triggered run-off from the Bantad mountain range, with the Trang River swelling.
About 2,500 families in 22 tambons of five districts have also been affected.
In Chumphon, gale-force wind damaged more than 100 houses and rubber farms in Pathiu district.
Residents along the banks of Khlong Chumko and Khlong Bang Son were advised to prepare to leave as water levels in the canals continued to rise.
The Asian Highway was flooded in spots in Lang Suan and Thung Tako districts, but traffic could still get through, the highway office said.
Meanwhile, State Railway of Thailand governor Prapas Chongsanguan said the track between Laem Tanod and Ron Phibun in Nakhon Si Thammarat was under 20cm of water, forcing south-bound trains to stop at Thung Song.
More than 1,000 passengers were earlier stranded in Nakhon Si Thammarat and Phatthalung due to flash floods.