On Friday, the High Court refused to grant leave, meaning Milat will have to serve at least 30 years in jail.

Milat, the great-nephew of infamous backpacker murderer Ivan Milat, killed David Auchterlonie with a double bladed axe in the Belanglo State Forest, the site of his great-uncle's deadly crime spree years earlier.

Milat's barrister John Stratton, SC, told the court that Milat did not receive a reduction in sentence for pleading guilty, whereas his co-offender, Cohen Klein, did.

Outside the court, David's grandfather and namesake David Auchterlonie said he was "frightened" that Milat would receive a reduction in his sentence as Klein had in the NSW Court of Criminal Appeal in March this year.

Klein, who was 18 years and two months at the time of the murder, had his sentenced reduced from a minimum 22 years and maximum 32 years to a non-parole period of 20 years and a maximum of 27 years.

"I was frightened ... but justice prevailed; that's all we can hope for," he said.

Mr Auchterlonie became emotional when he said time did not diminish the memories of his grandson.

"They're always with you. You never forget. He'll be 21 this year."

David's grandmother Sandra Auchterlonie said she clapped when Chief Justice Robert French announced that special leave to appeal had been refused.

"I clapped, it was just. I couldn't help it, I just clapped," she said.

Milat and Klein lured David into the forest, south of Sydney, with the intent of killing him.

Milat hit him in the torso with a double-edged axe and, as David pleaded for his life for more than 10 minutes, Milat threatened him, made him lie on the ground and hit him twice more before finally killing him with a blow to his skull.

Klein recorded the horrific events on his mobile phone.

Milat, who was aged 17 years and 11 months at the time, later bragged about the murder to his mates, telling them "You know the last name Milat ... I did what they do."

In June 2012, he was sentenced to a non-parole period of 30 years with a head sentence of 43 years.

Mr Stratton argued that trial judge Jane Mathews used her discretion to award Klein a discount for an early guilty plea but declined to do the same for Milat, which is against the policy of parity.

While conceeding the murder was a "very bad case", both Milat and Klein had only prior driving-related convictions and both pleaded guilty in the Children's Court, Mr Stratton said.

But Justice French said Justice Mathews decided the utilitarian value of Milat's early plea was outweighed by the extreme nature of his crime.

Mr Stratton went on to say that Justice Mathews had determined Klein should get 88 per cent of Milat's sentence, but in allowing him a discount, this reduced to 74 per cent.

The Court of Criminal Appeal's decision further reduced it to 62 per cent.

But after hearing Mr Stratton's submissions, Justice French did not wait to hear from the Director of Public Prosecutions Lloyd Babb, SC, before refusing to grant special leave.

"As soon as he said to the DPP not to get up, I knew we had him," Mrs Auchterlonie said outside the court.

David's mother, Donna Lock, also welcomed the outcome, saying she had been "hoping and praying that [special leave] was going to be refused". 

"I'm just really happy with the outcome because they should accept their sentence just like we had to accept what happened," Ms Lock said.

"David's never coming back." SMH