Simon Peter Carman was arrested at Bangkok's main international airport on Friday while allegedly attempting to leave the country on a Jetstar flight to Perth.
The 45-year-old was detained after the body of teenager Tunchanok Donhomla was found in a suitcase dumped next to a railway in the popular tourist destination of Pattaya, south of Bangkok.
Khanittha Thongsuknmag said she crossed paths with the alleged killer on June 5 when they were involved in a motorbike accident outside the apartment complex where they were both staying.
Recounting their interactions following the accident, she described a man she described as "King Kong".
"He was angry, angry," the 53-year-old said.
Messages between the pair, seen by AAP, show Carman telling Thongsuknmag to "buy me a wheelchair now ... or I will make sure big trouble".
When Ms Thongsuknmag tried to apologise and told him her insurance would cover the costs, he called her "stupid and dopey", according to the WhatsApp exchange.
"I have to see specialist in Pattaya. I need taxi!! you can pay, you evil BITCH," another message said.
Ms Thongsuknmag said she paid Carman 2000 baht ($A87) at the behest of police, but he continued to demand more money for a wheelchair.
"I blocked his number," she said.
Ms Thongsuknmag's boyfriend Andro said they were in disbelief when news broke of Carman being the suspect in Tunchanok's death.
CCTV from the early hours of Thursday morning showed Carman and the teenager returning to his apartment hand-in-hand.
It was there that she was strangled, police allege.
Footage later captured him allegedly leaving the property alone while pulling a large suitcase.
The property is minutes from Pattaya's Jomtiem beach, a popular tourist area known for its watersports and nightlife.
Locals said Carman often frequented a billiard bar in the Rimhad Jomtien block where he lived, but he never drank while there.
A woman at a restaurant, who asked not to be named over fear of the impact on her business, said she cooked food for him almost every day and noticed a big scar behind his head.
Carman told her he had surgery for a brain tumour, she said, and that was the reason for his slow speech and gait.
In the early hours of Saturday morning, police recovered the suitcase off a remote road near the Four Regions Water Market, discovering Tunchanok's naked body inside.
A handful of flowers lay in the overgrown grass down an embankment where the suitcase was found, with a water bottle and three bowls of food laid nearby.
Blue medical gloves and swab wrappers were strewn close to the busy road above.
Police footage showed the Ballarat-born man, who had visible scratch marks on his neck and arms, expressing regret to the girl's family.
"I feel bad for what happened to your daughter - it was out of my control," he said.
"It shouldn't happen. Please tell other girls just to be careful."
Separate footage showed him denying that he killed the teenager.
Carman could face life in prison or the death penalty if found guilty.
WA Police Commissioner Col Blanch said the Australian would likely have avoided the possibility of the death penalty if he had successfully boarded his flight back.
The Department of Foreign Affairs and Trade is providing Carman consular assistance.
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