Australian Simon Peter Carman "lived his normal life", going out, washing his clothes and talking to neighbours after allegedly murdering a teenage girl and stuffing her body inside a suitcase in Thailand, police allege.

Warning: This story contains graphic descriptions that some readers may find distressing.

Mr Carman, 45, was arrested at Bangkok's Suvarnabhumi Airport on Friday, just as he was preparing to board a flight to Australia, after a friend of 17-year-old Tunchanok Donhomlastruck reported her missing to police on Thursday.

The teenage girl's naked body was discovered by police in black luggage in waist-high grass alongside a railway line, about 10 minutes' drive from the apartment complex where Mr Carmen had been staying.

"After the girl [went] missing, … we suspected that something bad might happen," Pattaya City police colonel Anek Srathongyoo said.

Police alleged Mr Carman picked up Ms Tunchanok from the Beach Road vice strip in Pattaya in the early hours of Thursday morning, local time, and took her back to his complex in Jomtien.

CCTV video shows the pair walking hand-in-hand in the building's lobby at 3:34am before they both entered an elevator.

Ms Tunchanok, who was known as "Cake" to her friends, was never seen leaving the building. An unidentified friend later told officers she had lost contact with Ms Tunchanok shortly after the girl told her she had arrived at the building with a "foreign man".

Mr Carman, who lived in Western Australia before arriving in Thailand, told police he had not known the girl before they met in Pattaya.

After arriving at the complex, the pair went back to Mr Carman's room and got into an argument, police said.

Thai media is reporting Mr Carman and Ms Tunchanok fought over money, after having initially agreed he would pay her 1,000 baht ($43). They reportedly argued after he offered 500 baht instead.

Police said Mr Carman told them the girl "used a knife to threaten him" while they were arguing.

"He confessed that he killed her after they had an argument," police colonel Anek Srathongyoo said.

He said Mr Carman told them he squeezed her neck, causing her to lose consciousness, and then kept her body in the bathroom until later that night.

"After the incident happened, he just lived his normal life," Mr Srathongyoo said.

"Going out from the condo room, getting his clothes washed, chatting with surrounding people, no clue that he had done something bad."

About 9:25pm that same day, CCTV footage captured Mr Carman dragging a large suitcase out of his room and leaving the condo on a motorcycle with the suitcase strapped to the back.

He then sped alongside the railway tracks, disappearing from view for nine minutes. Footage shows him returning to the accommodation about 10pm.

Mr Carman has been charged with four offences: murder, concealment of a body, moving or destroying a body, and taking a minor aged between 15 and 18 for sexual purposes.

If found guilty, he could face between 15 and 20 years in jail, life imprisonment or the death penalty, police said.

Australian man grew up in Western Australia

Mr Carman grew up in Jerramungup, a small agricultural town in Western Australia, before moving to Albany as a teenager with his parents.

He later became a truck driver, but was involved in a motor incident, which caused an injury to his neck, two people close to him told the ABC.

The medication he took for the injury appeared to affect his speech and movement, people familiar with him said.

He had previously owned firearms, but WA court records show his guns were seized and his firearms license suspended by police in 2023.

He was last seen in Western Australia in October 2025 and has been in Thailand for the last eight months.

Mr Carman's most recent online history shows him commenting on photos and videos of women.

In one comment posted in October 2025, he wrote he was living in Thailand.

The ABC understands Mr Carman is now in detention in Pattaya Prison.

In a video after his arrest, Mr Carman addressed the victim's family, saying he felt "bad" about "what happened to your daughter".

Thongchai Donholma, Ms Tunchanok's father, is set to hold a ceremony near the train station where her body was found south of Bangkok later today.

"I am deeply saddened," he said of his daughter's death while outside a police station in Pattaya on the weekend.