Atikom Tanabat, from a committee to develop triangle economic area in Thailand's upper south, said he and his team went to inspect Takrut Island, which is located in Thai territory connected to Myanmar's Kaw Thaung.
He said local residents informed Thai authorities of the ships.
An initial inspection found that the large ships were from Indonesia, drilling sand in the Andaman Sea to send to Singapore for further silica distillation.
Workers on the ships speaking the Yawi language said about 1 kg of silica was distilled from 2,000 cubic metres of sand.
Silica is currently priced at Bt32,000/kg, used in electrical equipment and sanitary ware manufacturing. Singapore will use the rest of the sand leftover from the distillation for land reclamation, they said.
Mr Atikom said many local residents are worried that the sand drilling process will be the cause of future problems in the coastal area, as well as affect the ecological system.