Jail term slashed for Mitchell Barbieri who murdered Inspector Bryson Anderson

Inspector Bryson Anderson was stabbed to death in Sydney's north-west. Photo: Kylie Pitt

"I thought I was protecting my family and I have taken somebody away from theirs."

That is what Mitchell Barbieri told a psychiatrist after he murdered Inspector Bryson Anderson during a violent stand-off in 2012.

Barbieri, 21, was sentenced to a maximum of 35 years behind bars, with a non-parole period of 26 years, for murdering the decorated officer in Sydney's north-west.

His mother, Fiona Barbieri, was jailed for manslaughter.

The court heard that his mother was suffering from paranoid schizophrenia at the time and Mitchell Barbieri had experienced "a transferred delusional disorder" from her.

He appealed against his sentence, in part arguing that his mental health issues were not properly considered.

In the NSW Court of Criminal Appeal on Monday, Barbieri had 11 years cut from his non-parole period. He was re-sentenced to at least 15 years' jail, with a maximum term of 21 years and three months.

Justice Derek Price was the dissenting judge, proposing he be re-sentenced to a maximum of 32 years.

Barbieri's barrister Janet Manuell, SC, told the court earlier this year that Barbieri had become absorbed in the delusional beliefs of his mother.

"There is no question that the applicant was entirely brought up in his mother's delusions," Ms Manuell said.

Barbieri was as sick as his mother and held the same deluded, fanciful, suspicious and paranoid beliefs, the court heard.

Barbieri was barricaded inside his Oakville home with his mother when police arrived after a call from neighbours on December 6, 2012.

The mother and son screamed at police to go away, telling them to f--- off, 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 mallett, 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.