Later that night, the skilled cellist played a concert at the entertainment centre and acted "quite normally", though some people soon noticed unusual scratches and marks on his skin.

The body of 18-year-old Lyndsay Van Blanken, an apprentice Disney animator with "considerable humour and flair", was not found for six weeks, until January 2004 when residents in the Queens Park units reported a strong smell.

Matheson, who had grown intense and obsessive over Ms Van Blanken when she ended their relationship and became engaged to an American man she met online, was sentenced in 2006 to at least 18 years' jail, with a maximum term of 27 years, for her murder.

Nine years later, an unrelated High Court decision has allowed Matheson to argue acting Justice Jane Mathews made a legal error when taking a "two-step" approach to his sentence.

The Court of Criminal Appeal on Friday cut his maximum sentence by two years, finding the mistake - known as a "Muldrock error" - had occurred when the judge considered Matheson's pre-meditation both as an aggravating feature and a factor contributing the to the offence being "above the mid range" of seriousness.

But Justice Clifton Hoeben​ said the method of killing was "brutal and cruel", and that the 18-year non-parole period should stand, in a judgment agreed to by Justices Lucy McCallum and Peter Hall.

Police questioned Matheson when Ms Van Blanken went missing, and he said he met her near Bondi Junction station, but they parted ways at Queens Park.

After Ms Van Blanken's body was found in a cricket bag, Matheson's father told them his son had bought a sports bag in November.

Police discovered only one such bag had been purchased in the eastern suburbs in the previous six months.

Matheson was later diagnosed as possibly being schizophrenic, but acting Justice Mathews found whatever psychiatric disorder he was suffering had little to do with luring Ms Van Blanken and strangling her with cable ties.

The 21-year-old was placed in protective custody, where he was since completed art and literature courses.

In an affidavit to the appeals court, Matheson said: "I think that I have now taken a more peaceful and pragmatic approach to life.

"As an example, I remember that I was greatly depressed when I came into custody and could not play the cello.

"Before coming to custody, music and playing the cello was my life. I now accept that I exist as I exist and while I am in custody that means that I don't have the same access to music."

Matheson is eligible for parole in May 2022, and his new maximum sentence expires in May 2029.