The Director of Public Prosecutions has failed to get special leave to appeal to the High Court against the reduced jail sentence given to Mitchell Barbieri, who murdered NSW police officer Bryson Anderson.
On Friday, the High Court refused special leave, making its decision without even waiting to hear from lawyers representing Barbieri.
Supporters of Inspector Anderson's family gasped and cried and one person called out "joke" when the decision was announced.
Barbieri, 21, was originally sentenced to a maximum term of 35 years behind bars, with a non-parole period of 26 years for murdering the decorated officer in Sydney's north-west.
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The NSW Court of Criminal Appeal cut 11 years off his non-parole period in December.
He was given a maximum term of 21 years and three months with a minimum of 15 years.
The Court of Criminal Appeal's decision was described by Inspector Anderson's family as "beyond belief" and outraged the then NSW police commissioner Andrew Scipione
Mr Scipione wrote to the Director of Public Prosecutions, Lloyd Babb, SC, to pursue an appeal in the High Court.
During the hearing on Friday Mr Babb told the High Court the two appeal judges who cut the sentence made "a very significant error", which has "importance for development of the criminal law in NSW ... and is important for the administration of justice in NSW".
Mr Babb argued the appeal court was wrong when weighing up the need for general deterrence with the reduction in a person's moral culpability for a crime by reason of mental illness.
"Great weight must be given to the protection of the police force," Mr Babb said.
Sitting in Canberra and appearing via video-link to Sydney, one of the High Court justices asked Mr Babb of Barbieri's mental illness: "Did it not diminish to a very significant degree his moral culpability? It did. He was delusional."
But Mr Babb said Barbieri's decreased moral culpability should not have caused his jail term to be reduced by such a large amount.
He said a forensic psychiatrist, Dr Michael Diamond, gave evidence that Barbieri "knew the difference between right and wrong".
Barbieri was barricaded inside his Oakville home with his mother, Fiona Barbieri, when police arrived after a call from neighbours on December 6, 2012.
The mother and son screamed abuse at police, telling them to go away, and sent emails to a host of politicians including then prime minister Tony Abbott and Russian President Vladimir Putin, about having the right to defend themselves.
When police tried to storm in through the back door, Barbieri picked up a large hunting knife and stabbed Inspector Anderson twice.
His mother then ran outside, armed with a mallet, and attacked one of the officers trying to subdue her son.
She was originally charged with murder but the Crown accepted a plea to the lesser charge of manslaughter in 2014.
She received a maximum sentence of 10 years' jail with a non-parole period of seven years and six months.
Her son pleaded guilty to Inspector Anderson's murder and in contrast received a larger sentence.
He appealed against his sentence, in part arguing that his mental health issues were not properly considered.
The NSW Court of Criminal Appeal heard that Barbieri's mother was suffering from paranoid schizophrenia at the time and Barbieri had experienced "a transferred delusional disorder" from her.
Barbieri will be eligible for parole in December 2027.