A relative of Australian serial killer Ivan Milat, who murdered seven backpackers in the 1990s, was Friday jailed for at least 30 years for axing a teenage friend to death for a "thrill".

Matthew Milat, who pleaded guilty, was sentenced for killing the boy in the same forest in New South Wales state in which his great-uncle dumped the bodies of his young British, German and Australian victims.

The 19-year-old's accomplice, Cohen Klein, who also pleaded guilty to murdering David Auchterlonie on his 17th birthday, was jailed for at least 22 years.

"This was a thrill kill on the part of (Matthew) Milat," said New South Wales Supreme Court judge Jane Mathews of the murder, which was recorded on a mobile phone.

"He took the life of an innocent young man who was unfortunate enough to be his friend, in an unimaginable, cruel, brutal and violent manner solely for his own personal enjoyment."

The court heard Milat had said he was going to commit a murder, but his friends assumed he was joking.

Klein and another teenager, who has not been charged, drove with Milat and Auchterlonie into Belanglo forest in November 2010.

Milat at first swung a double bladed axe into the victim's torso and accused him of telling people about a theft he had committed.

Auchterlonie was then told to lie face down on the ground as the killer threatened him for 10 minutes, saying: "You move, I chop your head off."

The phone provided chilling audio of the victim sobbing and begging "don't do this to me" and the sound of an axe hitting him.

Psychiatrists testified that poetry written by Milat after the murder, and sent to his mother, glorified the crime and conveyed a sense of enjoyment in the killing.

The judge said Milat gloated about the murder, saying: "That's what the Milats do". She added that in the poems "he almost appeared to be revelling in the events".

His great-uncle Ivan was sentenced to seven life sentences in 1996 over a string of backpacker murders that terrified Australia.

He remains in jail after being convicted following sensational evidence from Paul Onions, a British hitchhiker who escaped death in 1990 by jumping out of Milat's vehicle when he pulled a gun on him.

Onions reported the incident to police at the time and years later when he saw news reports of bodies found in Belanglo, contacted the Australian High Commission in London, eventually providing crucial evidence at the trial.

Auchterlonie's mother told reporters outside the court she was glad Matthew Milat got as long as he did.

"It will never ever be truly enough because David is gone, and (Milat) can be released in time and go and live some of his life," she said.

"But I'm just glad that he didn't get any less."

Grandmother, Sandra Auchterlonie, added of Milat: "He makes me sick. He is just so cold blooded, callous and heartless."