A friend also reported seeing the same man checking up on her as she left the bar that evening.
The girl left the island following her assault on June 26 and attempted to report her ordeal to the Koh Phangan police. Officers reportedly refused to take any details of the alleged rape.
The girl's mother contacted the Fishbowl Bar and was told the CCTV cameras were not working on the night of the assault.
Fears over the safety of the idyllic Koh Tao island were heightened after the murder of Hannah Witheridge, 23, from Great Yarmouth, Norfolk and David Miller, 24, from Jersey in 2014.
Four other Western tourists, including three more Britons, died on the island in the two years following their deaths and the family of some of the deceased accused local police of not investigating the deaths properly.
Koh Tao has become a huge draw for young backpackers thanks to its stunning beaches and cheap accommodation - so much so that as many as 750,000 Britons visit each year.
But on September 15 2014, the island's community was left reeling after the battered bodies of Ms Witheridge and Mr Miller were found on Sairee Beach.
Ms Witheridge had been savagely raped and beaten to death and Mr Miller had been beaten unconscious and left to drown in the incoming tide.
Zaw Lin and Wai Phyo were sentenced to death for the killings. But although Mr Miller's family called the verdict "justice", the trial of the two Burmese migrant workers was plagued by accusations of human rights abuses.
The men, apprehended soon after the pair's bodies were found, initially confessed to the crime only to retract their statements amid claims they were extracted by torture.