The confession of Tiahleigh Palmer's foster brother to his mother that he'd had sex with the slain schoolgirl was a "catalyst" for her death, a court has heard.

Trent Thorburn, 20, was sentenced to four years' jail at the Beenleigh District Court today after pleading guilty to two counts of perjury and one count each of incest and attempting to pervert the course of justice.

Trent Thorburn's sentence will be wholly suspended for a period of five years on January 20, 2018.

Cindy Palmer, the biological Kiwi mother of murdered schoolgirl Tiahleigh Palmer, speaks to the media outside the Beenleigh District Court in Brisbane. Photo / AAP
Cindy Palmer, the biological Kiwi mother of murdered schoolgirl Tiahleigh Palmer, speaks to the media outside the Beenleigh District Court in Brisbane. Photo / AAP

The court heard Trent Thorburn, then 18, had sex with Tiahleigh at least once during the weeks leading up to her death in late October 2015.

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The victim's biological mother Cindy Palmer, who is a Kiwi, was seen outside Beenleigh District Court after the sentencing.

The 12-year-old's body was located on the banks of a northern Gold Coast river in November 2015, six days after police allege she was murdered by her foster father Richard Thorburn.

Crown prosecutor David Nardone said Trent Thorburn, fearing Tiahleigh was pregnant, had confessed to his mother Julene he'd had sex with her.

"His confession to his mother was the catalyst for Tiahleigh's murder," Nardone said.

Trent Thorburn, 20, pleaded guilty to two counts of perjury and one count each of incest and attempting to pervert the course of justice. Photo / Facebook
Trent Thorburn, 20, pleaded guilty to two counts of perjury and one count each of incest and attempting to pervert the course of justice. Photo / Facebook

The court heard Trent Thorburn had also admitted his sexual relationship with Tiahleigh in a Facebook messenger conversation with his cousin before confessing to his mother.

Following Tiahleigh's death, Trent Thorburn repeatedly denied having had sex with her to police as well as denying he knew anything about her murder. Thorburn then maintained those lies to a Crime and Corruption Commission (CCC) hearing in June 2016 before admitting his sexual relationship with his foster sister during another CCC hearing in September.

He did however continue to deny knowing anything about Tiahleigh's disappearance or death in that second hearing, Nardone said.

Slain schoolgirl Tiahleigh Palmer. Photo / News Corp Australia
Slain schoolgirl Tiahleigh Palmer. Photo / News Corp Australia

During that second CCC hearing, Trent Thorburn alleged Tiahleigh had initiated the sexual act and he was unable to resist her advances due to being under the influence of alcohol, a proposition Judge Craig Chowdhury dismissed as "completely implausible".

The sentencing hearing continues.