The family of a slain NSW police officer has condemned a High Court decision not to hear an appeal against a ruling that slashed the sentence of the mentally ill man who killed the detective.
The NSW Court of Criminal Appeal in December cut Mitchell Barbieri's minimum jail term from 26 years to 15 over the 2012 stabbing murder of Detective Inspector Bryson Anderson.
The NSW Director of Public Prosecutions applied for leave to challenge the reduction but on Friday, three High Court judges refused the application, saying the case didn't involve any error of legal principle.
"It's a result that defies description, really," the slain officer's brother, Warwick Anderson, said outside the Sydney court.
"If our system considers that is an appropriate sentence ... I'd invite the premier and the attorney-general to give that some serious consideration because it's not what your average person expects."
The casket of NSW Police officer Detective Inspector Bryson Anderson is carried into St. Patrick's Cathederal in Parramatta in 2012. Source: AAP Image/Dean Lewins
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 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 appeal court in December found the original sentencing judge erred when considering the severe mental illness of Barbieri, who'd pleaded guilty to murder.
It said it made no difference that his illness was secondary to that of his mother's, or that he recovered after being separated from her.
"His mental illness diminished his moral culpability to a very significant degree," Justice Carolyn Simpson said at the time.
Police Association of NSW acting president Tony King said his members were "disgusted" by Friday's decision.
A portrait of NSW Police officer Detective Inspector Bryson Anderson on display during his funeral. Source: AAP Image/News Ltd pool, Craig Greenhill
It suggested crimes were prosecuted in a legal rather than a justice system, he said.
Warwick Anderson said it was tough to expect police officers to put themselves in harm's way when "this is the result when people murder them".
He also said the fight was "not over".
"We look forward to having some meaningful discussion with the attorney-general about how repetition of this can be avoided," Mr Anderson said.
Fiona Barbieri was jailed for at least six-and-a-half years for Det Insp Anderson's manslaughter.