A 17-year-old boy was convicted on Thursday of killing a couple and their teenage daughter on their farm near Griekwastad, in the Northern Cape.

The Northern Cape High Court also found him guilty of raping the girl and of defeating the ends of justice.

"I am satisfied that the minor accused committed all the offences," Northern Cape Judge President Frans Kgomo said as he handed down judgment.

Kgomo said the State had proved the charges beyond reasonable doubt.

"The murders were premeditated."

Deon Steenkamp, 44, his wife Christelle, 43, and daughter Marthella, 14, were shot on their farm Naauwhoek on April 6, 2012.

Kgomo held that the teenager raped the girl and lied to the police. He said the girl was tortured before her parents were killed execution style.

Kgomo said he wanted to refrain from saying anything of the boy's demeanour in the witness stand, but remarked that he did not show any emotion. He testified in a strong voice and he did not become flustered when the State painted him into a corner.

The State told Kgomo the boy's bail fell away automatically following the guilty verdict. Prosecutor Hannes Cloete said the State would submit at least two reports before sentencing. One would be a report from a police psychologist and the other a victim impact report.

The boy's lawyer Riaan Bode submitted he would only take instructions on a possible application for extending the bail to sentencing during an adjournment.

Kgomo adjourned the sitting to Thursday afternoon, and told the boy he was in custody.