Many communities in low-lying areas on both sides of the Trang River have been flooded, with some houses nearly submerged, following the collapse of the river embankment in two places.
Dawn broke over Trang to see many parts of the town and its suburbs under water.
Among the hardest hit was Chulaphorn Ratchawithayalai Trang School near Khlong Chang bridge between tambons Bang Rak and Na To Ming in Muang district. Many houses nearby were flooded to the rooftops.
People have moved their belongings by boat to main roads, where temporary shelters have been set up.
In Muang district, about 1,600 people have been evacuated from riverside areas.
Sompis Kongthong, 66, a resident of tambon Bang Rak, said her house was totally under water, forcing the entire family to evacuate.
The road in front of Wat Prasitchai and Thessaban 1 School was about 1 metre under water.
In Narathiwat, seven tambons of Muang district, home to 8,042 households and 32,000 people, were also heavily flooded.
The Meteorological Centre for the South's eastern coast announced in its forecast at 6.30am on Sunday morning that rain would continue to hit Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Pattani, Yala and Narathiwat during the next 24 hours as a result of the northeast monsoon lying across the Gulf of Thailand and the South.
The disaster prevention and mitigation department of Songkhla and the provincial irrigation office were using about 10 water pumps and eight water propulsion machines to drain water out of heavily-flooded Ranot district into the Gulf of Thailand.