Authorities on Koh Chang island are stepping up efforts to stop wastewater discharges and encroachment on public areas at Hat Sai Khao.
Trat deputy governor, Prathan Surakitbawon, said he learned of the problem after the public lodged complaints with the Interior Ministry's Damrongtham Centre in Trat province.
Locals complained about polluted water near the beach, on a footpath, on a road, and the illegal parking of vehicles.
During a visit to Hat Sai Khao yesterday, Mr Prathan and officials noticed "traces" of untreated water released by some businesses in waterways.
The water had turned black and was giving off a strong stench.
Mr Prathan told officials to work with police and soldiers to find the source of the wastewater.
Immediate action is needed to prevent the pollution from spreading as the waterways are near Hat Sai Khao, a well-known beach on Koh Chang, said Mr Prathan.
The deputy governor also ordered officials to stop illegal parking along an 800-metre section of road where hotels and motorcycle rental shops are located.
When the governor inspected the area he noticed that some motorcycles for rent were taking up parking bays and chairs had been placed in roadside parking spaces to keep the bays free for customers.
Authorities have given the shop owners until the New Year to fix the illegal parking problem, he said.
Business owners will be banned from using chairs to block parking bays and motorcycle rental shop owners will only be allowed to park three motorcycles at certain spots, said Koh Chang mayor Sanya Koetmani.