Perth man Simon Peter Carman was arrested in June after Tunchanok Donhomla was reported missing in Pattaya, a city about 150km southeast of Bangkok known for tourism and its sex trade.
The suitcase containing her body was found soon afterward by railway tracks in a deserted area.
Carman, who turns 46 later this month, was brought in shackles to the Pattaya Provincial Court on Monday for a procedural hearing to designate witnesses to testify.
He has been charged with premeditated murder - which carries the possibility of the death penalty - two counts of indecent behaviour involving a minor and concealing a corpse.
Carman had initially denied all charges, saying that he was acting in self-defence and that he did not know the girl's real age.
However, when advised by a judge on Monday that the evidence against him on hiding the victim's corpse was strong, but he could expect leniency if he confessed to that charge, he indicated that he would change his plea to guilty.
Speaking to reporters ahead of the hearing, the victim's aunt, Miruntree Thanachai, said it would be the first time the family would meet the Australian defendant.
"We previously asked the authorities, but they didn't allow us to see him. We want to meet the person who killed our family member," she said.
Police said the girl was last seen on security footage walking into a condominium with Carman.
He was later seen leaving the building alone with a large suitcase. Security footage showed him loading it onto a motorbike and driving away.
The family said Tunchanok had gone on a vacation to Pattaya from their home in the rural northeastern province of Kalasin.
Police arrested the suspect as he was about to fly home to Australia.
They said subsequent questioning led them to the suitcase.
In a police interrogation video shared online by Thai media, Carman said he was defending himself because the girl was threatening him with a knife.
The victim's father, Tongchai Donhomla, told reporters that he and other family members had viewed the victim's body and that her face had been battered.
"Simon claimed that it was self-defence, but this is not self-defence because if it is self-defence, how can my child's teeth fall out? I want to ask," he said.
"I want the death penalty, but there is nothing I can do. I want the law to punish, to the fullest."
His sentiment was echoed by Miruntree Thanachai, one of the girl's aunts.
"The family wants the court to sentence him to death because we have lost our family member, the important one. So we want the execution because we don't want this to happen to others again. No one can bear this kind of damage," she added.
On Friday, the grieving family also filed a case against Carman in civil court, asking for damages of 1.65 million baht (about $A70,000)
