An 80-year-old Canadian woman has been found dead on Kata beach on Phuket, with the cause of the death initially said to be heatstroke.

A passer-by told police he first saw the tourist sunbathing with her face down at 10.30am on Wednesday. When he next checked at 1.30pm, he saw that she had not moved and her body had started to show some green bruises.

He then notified lifeguards and police.

Police said there were no signs the woman had been physically attacked. A key card for the Kata Palm Resort in Karon district was found among her belongings. She had checked in at the resort alone on Monday and was due to check out on Thursday, police said.

A security camera at the resort showed her leaving her room at 10.23am on Wednesday, police said. She asked resort staff how to get to the beach. Police said she did not look sick in the video.

The forensic department of Vachira Phuket Hospital estimated the woman had died four to six hours before doctors conducted their examination, and that the initial cause was said to be heatstroke.

Police said an autopsy would be performed.