Tiahleigh Palmer's mother and the foster brother of the murdered schoolgirl have shared their sides of the tragic story, with Cindy Palmer saying foster mother Julene Thorburn left her daughter "there to die" at the hands of her foster father.

Rick Thorburn, 57, who last week pleaded guilty to murdering 12-year-old Tiahleigh and dumping her body on the banks of a Gold Coast river in October 2015, was sentenced to life in jail.

Ms Palmer has spoken of her devastation on the Nine Network's 60 Minutes program about when she learned her daughter had been murdered.

"I fell to the ground crying because I guess obviously it was my daughter and she was found washed up on a river — what kind of animal leaves a little girl in a river?" Ms Palmer said.

"But that pain I felt was so bad, I couldn't do anything — I couldn't do anything but cry."

'That pain I felt was so bad, I couldn't do anything … but cry," Cindy Palmer said. ( AAP: Darren England )

'Tiah is no longer with us'

In a police interview aired on the program, Josh Thorburn, 22, told of the moment he knew his father had killed his foster sister.

"His exact words were: 'Tiah is no longer with us'," Josh Thorburn said, recounting to police his father's conversation with the family.

"And also he said: 'I hope you understand what that means' — I knew what he meant — that he'd killed her."

Josh Thorburn was last year jailed for three months for lying to police. ( AAP: Glenn Hunt )

Josh Thorburn was last year jailed for three months for lying to police.

He pleaded guilty to perjury and attempting to pervert the course of justice, claiming he lied because of a secret family pact.

His younger brother Trent had had sex with the 12-year-old girl.

Trent Thorburn had feared Tiahleigh would be pregnant and he would go to jail.

In the police interview Josh Thorburn was asked how his father Rick Thorburn's emotions were at the time.

Josh replied: "He didn't cry — he seemed quite assured and defensive of what he'd done and why he had done it — obviously it was to protect Trent."

Josh Thorburn told police his father did not discuss how he killed Tiahleigh.

"No — I still don't know what happened there — I just know that he did do it," Josh Thorburn said.

"I know that he said to us 'that it's best that we don't know how it happened, because the less we know, the less we can tell'."

In a handwritten letter tendered to the court on Friday, Rick Thorburn said he still loved the 12-year-old girl whose life he brutally took and could not remember the night he murdered Tiahleigh or much of the two-and-a-half years that followed.

Julene, Josh and Trent Thorburn have now all spent time behind bars for lying to police and trying to cover up Tiahleigh's murder.

Rick Thorburn will not be eligible for parole until 2036.

Thorburn family (LtoR) - Rick, Josh, Julene and Trent. ( AAP/Supplied )

'Severe domestic violence situation'

Tiahleigh had been in foster care since she was seven years old and arrived at the Thorburn's in January 2015.

Tiahleigh's body was found on the banks of the Pimpama River on the northern Gold Coast. ( AAP/Qld Police )

Ms Palmer said she had been in a "severe domestic violence situation" when she went to the Department of Child Safety for help.

"I realised that one day I was just going to die and they [her children] were just going to be left there by themselves," Ms Palmer said.

"At first I was so devastated being apart from her [Tiahleigh] that I spiralled out of control into drugs and homelessness.

"I still firmly believed that the day I went to the department asking for help I made the right decision.

"It's not what I wanted, and it wasn't the outcome any of us wanted, but at the end of the day I thought that was the safer option."

But Ms Palmer said she had misgivings when Tiahleigh was fostered by the Thorburns, especially as she would living with teenage boys.

"Straight away I said no," Ms Palmer said.

Tiahleigh 'didn't want to be left alone with Rick'

Ms Palmer said she noticed her daughter's behaviour changed towards Rick Thorburn.

"She didn't want to be left alone with him and she started misbehaving, she was running away — she didn't want any bar of Rick," Ms Palmer said.

"It wasn't just that — it was the inappropriate clothes that she all of a sudden started wearing and dresses that had padded bras in them.

"He had taken her to get her eyebrows waxed and things like that."

Ms Palmer said Tiahleigh had run away before and was always found within a few hours.

But this time felt different, as she said she had done everything she could to help find her daughter.

"I think that my heart was telling me that I had to find her and that I wanted to find her and things were going to be OK, but I think deep down I knew that something was really wrong," she said.

'Stick to the same story'

Queensland police Detective Inspector Damien Hansen said in hindsight Rick Thorburn's behaviour was very emotional from the start.

"He's broken down in tears so much so that the social worker had to drive him to the police station to report her missing," he said.

Tiahleigh's body was found in the Pimpama River six days after she was reported missing.

"It's total disregard — it shows what he [Rick Thorburn] thought of her — they dump trash here — that obviously sums up what he thought of Tiah," Detective Inspector Hansen said.

"She was just off the bank … in the reeds, just down only in shallow water.

"She was naked apart from her underwear that was partially pulled down.

"She was decomposed so identification was the trouble."

Detective Inspector Hansen said police interviewed all the children at school as Rick Thorburn had told police he had dropped Tiahleigh off at 8:10am.

Police also canvassed all sex offenders in the area — 350 were in the area at the time.

They eventually received a tip-off to Crime Stoppers that alerted them to a Facebook message Trent Thorburn had sent to his cousin.

"I'm in trouble and I don't know what to do," the message said. "I have never had sex before and she [Tiahleigh] made me do that with her. "I just want the kid gone and out of my life but I know she is also a source of income for mum and dad as well."

Detective Inspector Hansen said they knew the family was putting stories together.

Police bugged the home at Chambers Flat, south of Brisbane, and heard how they planned to cover up the murder, with Julene Thorburn heard saying: "We will have to keep you out of this completely".

"We have to stick to the same story about her going to school the next day and whatever," she said.

Rick Thorburn coached his younger son Trent what to say to police: "She came into your room. You remember waking up. You're still drunk, you're still whatever," he had said.

'We've got to come clean'

But police also heard Julene Thorburn and Josh begin to crack.

"If we have to come clean, that's just between you and I," Julene was heard saying to Josh.

"We will have to come clean — we've got to come clean to protect you and me and Trent for the rest of it.

"Dad made the decision to go down that path unfortunately and we're going to have to live with it, if it comes to that."

Police put the Thorburn family before coercive hearings and were able to prove that Julene, Trent and Josh had all lied at those hearings, committing perjury.

It was a crucial moment in the investigation and used to leverage Julene and Josh to get them to confess and become Crown witnesses.

"I just knew that I couldn't keep doing this for very much longer because it was going to catch up with me," Julene Thorburn told police.

"I said to Rick: 'Was Tiah OK?' and he said: 'It's all taken care of'.

"I said: 'What do you mean by that?'

"He said: 'Don't worry, don't ask any questions, I've taken care of it'."

Julene said the last time she saw Tiahleigh, the 12-year-old was in her bedroom sitting on her bed.

"I said goodnight to her there and expressed to her that she needed to stay there and put herself to sleep when she was ready sort of thing," she said.

Rick Thorburn killed 12-year-old Tiahleigh in October 2015. ( AAP: Dave Hunt )

Rick Thorburn was outside the kitchen door.

Julene said she left the house then, but had no idea her husband would kill Tiahleigh.

"No — absolutely not, no —I would never have left the house if I thought that," she said.

"I had no indication, no feeling that he would do something like that at all."

Julene left Tiahleigh 'there to die', Cindy Palmer says

But Ms Palmer thinks differently.

"I believe that Julene left Tiahleigh there to die that night," she said.

"If we believe the story that they're telling — that she left to go be at her sister's house after the conversation happened about Trent sleeping with Tiahleigh — I believe she left her there to die."