FREDERICTON -- The trial judge who presided over Curtis Bonnell's murder case did not properly explain to the jury how the evidence they heard could support a manslaughter verdict, the man's lawyer argued Tuesday before New Brunswick's Appeal Court.

Bonnell was found guilty in November 2012 of first-degree murder in the death of his 16-year-old cousin Hilary. The offence carries an automatic life sentence with no parole eligibility for 25 years. He is appealing his conviction and seeking a new trial.

His lawyer, Peter Corey, said Tuesday that the trial judge in his instructions to the jury should have related the evidence they heard to a possible finding of manslaughter.

Corey said the trial judge also erred by allowing text messages Hilary Bonnell sent before she died in 2009 to be entered as evidence.

The jury was shown two text messages that Hilary sent to Haylie Bonnell, the accused's sister, on the morning she disappeared. They read, "Please answer me I'm scared," and "OMF text me I'm scared."

"There is nothing in them to identify who she's with or what she's scared of," Corey told a panel of three New Brunswick Court of Appeal justices.

Corey said the judge also failed to properly define the meaning of the term homicide. During the trial, the jury was told that autopsy and toxicology reports were inconclusive on the exact cause of death, but those reports termed the manner of death as a homicide.

Corey said many people assume that to mean murder.

But Crown prosecutor Cameron Gunn told the appeal court's justices that the trial judge gave clear and proper instructions to the jury.

"He makes no error," Gunn said, adding that the trial judge raised evidence related to various charges, including manslaughter.

Gunn said the judge was correct to admit Hilary Bonnell's text messages as evidence because they relate to her state of mind when she was with the accused.

"At the end of the day, Mr. Bonnell got a fair trial and was found guilty by a jury of his peers," Gunn said.

Pam Fillier, Hilary Bonnell's mother, attended the appeal hearing Tuesday. She said outside court that she was glad Curtis Bonnell was not there but it was still painful to hear the details of the case again.

"(Hilary) would have been 21 yesterday," she said. "I shouldn't be here the day after my little girl's birthday."

During the trial, the Crown alleged Curtis Bonnell picked up Hilary on a rural stretch of road near the Esgenoopetitj First Nation on Sept. 5, 2009, sexually assaulted her and killed her.

Curtis Bonnell was arrested on Nov. 8, 2009, and the next day led police to a remote site where the girl's body had been buried.

Police interviews were entered as evidence at the trial and they show Bonnell saying that he had sex with Hilary, fought with her, and killed her by holding his hand over her mouth to stop her from screaming.

But he offered a different version of events at his trial when he testified in his defence.

He said he woke up after a night of alcohol and drugs to find Hilary dead next to him in his pickup truck. He said he panicked and buried her in a remote wooded area.

The appeal court reserved its decision.