At least 27 people have been killed and many more are missing after flash floods swept over two waterfalls in southern Thailand.

The tragedy took place at the Sairung and Prai Sawan waterfalls where swimmers were taking advantage of the five-day holiday weekend for the Buddhist New Year in Trang province, south of the capital Bangkok.

"Many people were swimming when flash floods swept over the waterfalls," said a local disaster prevention official.

"The floods came so quickly. It happened in just 10 minutes.

"At least 27 people were killed and a large number of people are still missing," the official said, adding he was unable to give an exact number of those unaccounted for.

When the flash floods struck the waterfalls, which are about five kilometres apart, there were some 100 holidaymakers at the popular sites, he said.

The weekend crowd was bigger than usual due to the Buddhist New Year holiday, which began on Friday (local time), the official added.

"People were enjoying and relaxing at the waterfalls and they were totally caught off-guard by the floods," the official said.

Hundreds of rescue workers, including military personnel, local police and volunteers, were mobilised to hunt for the missing and to help survivors, a senior provincial official said.

"We are continuing our search and rescue operations," he said.

Most of the victims were women and children, a local hospital official said.

Several injured tourists were stranded on rocks and trees waiting for rescue workers, The Nation, an English-language newspaper, reported on its website.

The area had been hit by heavy rains since Thursday (local time), local reports said.