In the early hours of Thursday, they returned to his apartment, where she was strangled, police said.
Two days later, Thai police said they found the teenager's naked corpse, stuffed inside a suitcase and dumped in waist-high grass, near a railway track, a short distance away from the beachfront.
At almost the same time as the body's discovery in the early hours of Saturday, Thai immigration authorities at Bangkok's Suvarnabhumi Airport apprehended a middle-aged Australian man as he prepared to board a flight out of the country.
Now in custody, Simon Peter Carman has been charged with intentional murder, concealing a corpse, moving or destroying a corpse and abduction of a minor for indecent purposes, according to Thai police.
Reuters was unable to reach Carman for comment, and police said he had not yet appointed a lawyer.
Australia's Department of Foreign Affairs and Trade said it was providing consular assistance to an Australian detained in Thailand and declined to comment further.
In Pattaya, a popular beach resort known for its colourful night life and an epicentre of illicit sex tourism, police said Carman had made a partial confession.
"He said he did not intend to kill her, but admitted strangling her, causing her death," Police Colonel Anek Srathongyoo, the superintendent of Pattaya City police station, told Reuters.
"He stated that they had an argument and, during the altercation, he strangled the woman."
Authorities were first alerted of a missing person about 5pm on Friday when a friend of the victim reported to Pattaya City police station that she had disappeared after being seen walking with a foreign man, according to a police statement issued on Saturday.
But initial leads were thin. "All that was known was that the man took her to a condominium," the statement said.
Police then began scouring CCTV footage and searched an apartment where they found a passport of the suspect, but were unable to trace either the man or the girl.
It was not clear how Carman hoped to leave the country without his passport.
CCTV footage, released by police, shows a girl in jeans and a taller man in shorts and black sleeveless T-shirt holding hands and walking into a lift around 3.35am on Thursday.
"On the day of the incident, he was out walking when he met the victim," Anek said, referring to Carman.
"They struck up a conversation, decided to continue spending time together, and later returned to his condominium."
Reuters could not immediately determine why the girl agreed to visit the suspect's apartment.
International law enforcement and charities have long raised concerns about sexual exploitation in the area, particularly of children.
Following an argument that led to Carman allegedly strangling the victim to death, the suspect initially kept the corpse in the suitcase, only later taking it out of the building, police said.
A second CCTV video shows the man, dressed in similar clothes, dragging out a large black suitcase.
Reuters was able to confirm the location from the building, road layout, elevator and floor patterns, which matched archive and satellite images.
The suitcase was verified to be the same one found by Thai police later, containing the victim's body.
The autopsy results were still pending and investigators continued to gather evidence, which would be submitted to prosecutors who would decide on the indictment, Anek said.
The girl's stepmother, Oradee Bussarakum, said when they saw news coverage of the suitcase they feared the worst.
"We were scared," she said.
"We just hoped it wouldn't turn out the way we feared. Now our eyes are swollen from crying."
