Lawyers for a would-be serial killer found guilty of murdering a Perth teenager have argued in Western Australia's highest court that her conviction should be overturned because "inadmissible" evidence was allowed at her trial.

Key points: Jemma Lilley and her partner Trudi Lenon were convicted of killing Aaron Pajich

Jemma Lilley and her partner Trudi Lenon were convicted of killing Aaron Pajich Lilley's lawyers argue some evidence presented at her trial was not admissible

Lilley's lawyers argue some evidence presented at her trial was not admissible But prosecutors said this evidence was not used in isolation to prove guilt

Jemma Lilley, 27, and her accomplice, Trudi Lenon, 45, are currently serving life jail terms with a 28-year minimum for the murder of 18-year-old Aaron Pajich at Lilley's Orelia home in June 2016.

The teenager was lured there and then attacked with a garotte and stabbed three times before his body was buried in the backyard and covered with cement and brightly coloured household tiles.

Lilley has always maintained her innocence and today appeared in the Court of Appeal via video link from the Eastern Goldfields Regional Prison, as members of Mr Pajich's family sat in the public gallery.

Aaron Pajich was lured to a home in the Perth suburb of Orelia where he was murdered and buried in the backyard. ( Supplied: WA Police )

Argument centres on serial killer book

At their Supreme Court trial in 2017, both women claimed it was the other who murdered Mr Pajich, with Lilley saying she was asleep when he was garrotted and stabbed.

Today she took her case to three judges in the Court of Appeal where her barrister, Simon Watters, argued that evidence about a book his client wrote when she was 16 years old should not have been put before the jury.

The fictitious book, called Playzone, was about a serial killer called "SOS" and prosecutors argued it showed that Lilley had a "propensity" or a "tendency" to want to kill someone.

Mr Watters said the text of the book and the words spoken by the main character — which were written years before Aaron Pajich was murdered — bore no resemblance to his manner of death and therefore should have been ruled inadmissible.

"The book was written by someone as a 16-year-old," he said.

"It talks about a male serial killer and in effect is in past tense — what has been done, not what will be done.

"It is of no significant probative value."

The President of the court, Justice Michael Buss, noted that while the book may have been written years ago, it was not then "placed in the closet or on the shelf and never heard of again" saying there was evidence at the time of the killing that Lilley had been referring to herself as SOS.

An 'obsession' with knives

Another argument put forward by Mr Watters was that evidence about Lilley's collection of knives also should not have been allowed as propensity evidence against her.

Her trial was told that after Mr Pajich's murder police found dozens of weapons at her home, including machetes, scalpels and butcher's knives.

But Mr Watters argued that the details of the knives should not have been allowed as evidence to show Lilley had a propensity to kill.

Aaron Pajich's stepmother Veronica Desmond and his father Keith Sweetman were in the Court of Appeal during the hearing. ( ABC News: Joanna Menagh )

Justice Mazza quizzed Mr Watters about his submission, saying there were an extraordinary number of knives found and it was fair to say Lilley had an "obsession" with them and their use in violent circumstances.

The third ground of appeal centred on a picture of someone being sliced, which was shown to the jury while Lilley was being cross-examined by Lenon's lawyer.

Mr Watters said the picture was unfair and prejudicial to his client, and at that point the jury should have been discharged and Lilley given a separate trial.

Both Mr Watters and the prosecution, represented by WA Director of Public Prosecutions Amanda Forrester SC, had earlier provided written submissions to the court.

As a result the court heard only briefly from Ms Forrester, highlighting that the evidence about Playzone was just one aspect of the case and was not used alone to prove Lilley's guilt.

The court reserved its decision.