Joshua Thorburn was charged with one count of perverting the course of justice and perjury during an 11-month police investigation that led to Rick Thorburn, 56, being charged with Tiahleigh's murder.

On Thursday, the 21-year-old was sentenced to 15 months imprisonment after he pleaded guilty to the two charges.

He will serve three months before the sentence is suspended for three years.

The court heard he had provided false information relating to Tiahleigh's disappearance and death to police on two occasions during November 2015 and again at a Crime and Corruption Commission hearing seven months later in June 2016.

Murder-accused Rick Thorburn claims he dropped Tiahleigh off at Marsden State High School on Monday, October 30, 2015.

A missing-person alert was issued by police six days later, hours before the girl's decomposing body was found on the banks of the Pimpama River by fishermen.

The court heard on November 5, 2015, the day Tiahleigh's body was found and on the following day, Mr Thorburn signed a statement to police that Tiahleigh was in bed at home on the night of October 29 and that he saw her the next morning and exchanged greetings before she was taken to school by his father Rick Thorburn.

On November 29, 2015, the 21-year-old dancer took part in a three-hour interview with police and gave a statement "repeating those lies", Judge Craig Chowdhury said during his sentencing remarks.

"If you had have told the truth early on, many police hours and hundreds of resources could have been saved and a proper focus could have been made at an earlier time of the real killer of Tiahleigh," he said.

"You had time to think about your actions ... and you chose to deliberately lie."

Judge Chowdhury said Mr Thorburn told a "series of lies" during a Crime and Corruption Commission hearing on June 27, 2016.

The court heard Mr Thorburn had been driven to and picked up from the hearing by his father, which "may have added to the pressure on you", Judge Chowdhury said.

"While no specific threats were made to you, I accept that you were fearful legitimately of your father," he said.

The court heard electronic surveillance devices were placed in the Thorburn's home that recorded Tiahleigh's foster mother Julene encouraging the family to lie at the hearings, prosecutor David Nardone said.

"She told the family (they) have to stick to the same story about her going to school the next day," Mr Nardone said.

The court heard Rick Thorburn had called a family meeting on the night of October 29 and told the family about the alleged sexual intercourse between Trent and Tiahleigh and that they had to protect Trent.

"Joshua told police that his father then said "Tia was no longer with us", Mr Nardone said.

He said Joshua acknowledged that no one checked to see whether Tiahleigh was still at the home.

Defence barrister Jeff Hunter QC said the day of the family meeting had been "perfectly normal" for Joshua until he went home.

"One can only imagine what he thought when he was told by his father that there was to be a family meeting and no one was to have a mobile phone with them and he was then told about what his father had done, told about what his brother had done," he said.

"He was in a sense trapped by the lie and had to go along with it.

The court heard that Mr Thorburn senior did not directly threaten Joshua, he was was "nonetheless well and truly afraid of him", Mr Hunter said.

Mr Hunter told the court Joshua was a "frightened and bewildered 20-year-old" that night.

"My client instructs that immediately afterwards he was told not to go into Tiahleigh's room and so he didn't but he had to walk past her room on the way to his own," he said.

"He says that once he went to his room he sat there with his phone and agonised for a long time about what he should do.

"He was concerned about being heard if he called the police, he was concerned that someone else may be harmed and he was concerned about the complete destruction of his family unit.

"He wishes that he did the right thing immediately and he says that throughout the period that he gave or maintained this false account he felt dreadful about it."

Judge Chowdhury said there had been an "extraordinary amount" of publicity regarding Tiahleigh's disappearance and the subsequent finding of her body.

"At some point, this is what amazes me, that your client's conscience wasn't piqued to say that this is so bad I can't do this anymore," he said.

The court heard Mr Thorburn had experienced "social isolation" as a result of the offences, which had caused him to lose friendships, a relationship with a woman and his employment as a dance teacher.

Tiahleigh's mother Cindy Palmer told media outside court, surrounded by loved ones, the justice system had "let us down horribly today".

"But most of all they have let down Tiahleigh and her fight for justice," she said.

A spokesman for the Palmer family said they would be looking to appeal the sentence.