Thailand is bracing for a potentially devastating Tropical Storm Pabuk, which is set to bring heavy rains and high seas to the country's southern provinces and famed beach resorts popular with domestic and foreign tourists.

Weather authorities on Thursday said the storm was moving west from the tip of Indonesia into the Gulf of Thailand with maximum winds of 65 kilometres an hour.

Heavy downpours are forecast until Saturday in most parts of southern Thailand, including popular tourist destinations such as Phuket, Koh Samui and Krabi.

There has been no official evacuation order but tens of thousands of tourists have fled the resort islands of Koh Phangan and Koh Tao as tourists packed onto ferries bound for the southern Thai mainland, as swimming was banned and boats were set to suspend services.

"I think the islands are almost empty ... between 30,000 to 50,000 have left since the New Year's Eve countdown parties," Krikkrai Songthanee, Koh Phangnan district chief, told AFP news agency.

The acting mayor of Koh Tao, one of Southeast Asia's finest diving spots, said boats to Chumphon on the mainland were crammed with tourists, but several thousand guests were still on the island likely to brave the storm.

On Koh Samui, authorities said they were preparing shelters for any tourists who decide to wait out the storm.

High seas

Pabuk, which means a giant catfish in Lao, is Thailand's first tropical storm in the area outside of the monsoon season for around 30 years.

It was unlikely to intensify into a full-blown typhoon, according to forecasters.

"But we expect waves as high as five or seven metres near the eye of the storm. Normally in the Gulf of Thailand there are only two-metre high waves," Phuwieng Prakammaintara, director general at the Thai Meteorological Department, told reporters.

"It's difficult to predict the severity of the storm so people should comply with authorities' recommendations."

Pabuk is also expected to dump heavy rain across the south, including the southernmost provinces bordering Malaysia of Pattani, Narathiwat and Yala.