NSW's police commissioner says he's "devastated" the High Court will not hear an appeal against the slashed sentence given to Detective Inspector Bryson Anderson's mentally ill murderer.
Three High Court judges on Friday refused to grant the Director of Public Prosecutions leave to appeal last year's reduction of Mitchell Barbieri's minimum sentence from 26 years to 15.
Commissioner Mick Fuller said he acknowledged the court's decision but he would continue to champion harsher sentences for people who harm police officers.
"Like every police officer, I am devastated by the decision," he said in a statement on Friday.
"Bryson was the finest police officer. He was decorated and dedicated.
"He deserved to go home to his family at the end of his shift."
Barbieri had pleaded guilty to the 2012 murder but his sentence was substantially reduced in December after the NSW Court of Appeal found the original sentencing judge erred when considering his severe mental illness.
The High Court said there had been no error of legal principle in the appeal court's decision.
Det Insp Anderson's brother, Warwick Anderson, said it was hard to expect police officers to put themselves in harm's way if "this is the result when people murder them".
He said the fight was "not over".
"It's a result that defies description, really," he said outside the Sydney court on Friday.
"We look forward to having some meaningful discussion with the attorney-general about how repetition of this can be avoided."
Then 19-year-old Barbieri was sharing mental delusions with his mother, Fiona, when he stabbed Det Insp Anderson twice in the chest with a hunting knife during a siege at the pair's rural northwest Sydney property.
Police seized a large volume of material from the squalid, isolated home, among them letters to world leaders including the Russian president.
They revealed "a complex delusional belief system alleging corruption, persecution and perceived grievances of a wide-ranging nature", according to a psychiatrist.
The Court of Appeal in December said it made no difference that Barbieri's illness was secondary to his mother's, or that he recovered after being separated from her.
One of the judges, Justice Carolyn Simpson, said Barbieri's "mental illness diminished his moral culpability to a very significant degree."
Police Association of NSW acting president Tony King said his members were "disgusted" by Friday's decision.
It suggested crimes were prosecuted in a legal rather than a justice system, he said.
Fiona Barbieri was jailed for at least six-and-a-half years for Det Insp Anderson's manslaughter.