Thai police are investigating the suspected murder of a Swiss woman whose body was found near a waterfall at a popular beach resort.

Police found the woman, named as Nicole Sauvain-Weisskopf, lying face down covered in a black sheet, in water among rocks near a beauty spot on Phuket island on Thursday.

The case casts a shadow over Phuket's so-called "Sandbox" scheme, a pilot project to reopen Thailand's Covid-devastated tourism sector after more than a year of draconian travel curbs.

The 57-year-old victim had travelled to Phuket under the scheme, which allows vaccinated travellers to visit the island without going through Thailand's otherwise mandatory two-week hotel quarantine.

Results of an autopsy to determine the cause of death are expected later on Friday, but officials believe Sauvain-Weisskopf died at least three days ago.

Officers found the woman's phone, shorts and trainers near the body.

Phuket governor Narong Woonciew pledged to "do everything to investigate what happened and bring justice to Ms Nicole".

Andrea Kotas Tammathin, honorary Swiss consul in Phuket, lamented what she called "a sad, sad day for the Phuket community".

More than 16,000 people have arrived in Phuket since the July 1 launch of the Sandbox scheme.

Prime Minister Prayut Chan-O-Cha has expressed his condolences to the victim's family and asked to be regularly updated on the probe, a government spokesman said.