A teenage relative of backpacker killer Ivan Milat has been jailed for at least 30 years for the axe murder of a friend in bushland south of Sydney.

In 2010, David Auchterlonie was struck over the head with an axe in the Belanglo State Forest, the same place where Ivan Milat buried seven victims in the 1990s.

He was lured there on his 17th birthday.

Tortured: 17-year-old David Auchterlonie. ( www.facebook.com )

Two 19-year-olds, Cohen Klein and Matthew Milat - who is Ivan Milat's great nephew - pleaded guilty to his murder. Milat hit the victim with the axe while Klein made a mobile phone recording.

Milat was jailed for 43 years on Friday with a non-parole period of 30 years, while Klein was jailed for up to 32 years with a minimum sentence of 22 years.

Wearing suits, the teenagers sat side by side in the dock, avoiding the gaze of the victim's family and the packed public gallery.

David Auchterlonie's mother Donna Locke told reporters she accepted Milat's sentence.

"I'm glad he got as long as he did," she said.

"I will never ever be truly enough because David is gone and he can be released in time and go and live some of his life.

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

Detective Inspector Mark Newham worked on the case and says the sentence reflects just how horrific the crime was.

"All murders, all homicides are obviously terrible in their own way," he said.

"But this is certainly one of the worst we've come across and particular having to listen to that recording and knowing what David Auchterlonie would have suffered in those final minutes."

During the sentencing, Acting Justice Jane Mathews described how Milat chased Auchterlonie around a car which was parked in the state forest, after accusing him of spreading stories about him.

She said the sound of the axe striking the victim on the head was captured on the mobile phone recording.

Justice Mathews said Milat later gloated about the murder, saying: "That's what the Milats do."

She said Milat glorified the murder by writing poetry about it and read out one poem he wrote months after the murder called "Your Last Day".

The judge said Milat had shown no remorse and posed a "serious threat to the community".

She said his letter of apology to the victim's family was insincere, and that he was "revelling in the memory of this terrible event".

Justice Mathews said Auchterlonie was tortured for the last two minutes of his life in a cold-blooded, pre-meditated killing.

"This was a thrill kill on the part of Milat," she said.

In a victim impact statement read out at an earlier sentencing hearing, David Auchterlonie Sr, the victim's grandfather, said the teenage killers deserved no mercy.

He said they terrorised and tormented his grandson, and when he saw the body he could see the fear in his grandson's eyes.

David Auchterlonie's mother Donna Locke previously called for his killers to be jailed for life and rejected a handwritten apology from Milat.

The letter, which was tendered as evidence during the sentencing hearing, said: "Since I have been detained I have been spending a lot of time thinking about what I have done and pray that this is a bad dream and I will wake up and all this is just my imagination''.

Prosecutor Lloyd Babb SC had told the court the offenders had no motive for their crime, but got a thrill and an adrenaline rush from seeing someone murdered.