In mid-May, photos and videos of black-coloured wastewater were widely circulated on social media, raising concern over the environment of the famous beach town visited by thousands of tourists each day.

The black water comes from wastewater released by tourism operators in the town into a canal, which then flows into the sea with the help of rain water, said Paisarn Kunsri, head of Krabi's Disaster Prevention and Mitigation Department.

"The release of wastewater has gone on for a long time but the black water only surfaces this year," he added.

Local authorities at Ao Nang are scrambling to tackle the problem, mainly by cracking down on tourism operators without wastewater treatment equipment and increasing treatment capacity.

But tourists themselves will not be affected, as the wastewater was not found to be toxic, and there has yet to be a need to limit the number of visitors, according to Krabi's Natural Resources and Environment Office.

Just 40km south of Ao Nang, Maya Bay, another popular tourist hotspot best known as the filming location of 2000 Hollywood movie The Beach, has been closed to tourists since last June as overtourism almost destroyed all the coral reefs in the area.

Maya Bay will remain closed for two more years to allow new coral to grow.

A record 38 million tourists visited Thailand last year, with 41 million others expected this year.

Australian Associated Press