Fiona Barbieri was a successful finance worker and single mother whose complete descent into mental illness culminated in the murder by her son of a decorated police officer, a court has heard.
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Detective Inspector Bryson Anderson was fatally stabbed by Mitchell Barbieri during a siege at the semi-rural property he shared with his mother at Oakville on December 6, 2012.
During a sentencing hearing in the NSW Supreme Court on Tuesday, Fiona's barrister Grant Brady said the now 47-year-old had been a "capable woman" and a "high functioning" employee at American Express. She had no criminal record.
About 2008, she developed late-onset chronic paranoid schizophrenia and became seriously unwell.
The court heard Mitchell, who was aged about 15 or 16, regularly had friends to the house. Fiona would tell the teenage boys she was being spied on. She also began drinking and smoking cannabis with them.
The boys' parents also noticed that the Barbieri's property, in Sydney's north-west fringe, became neglected and the electricity was cut off.
Mr Brady said Fiona had deeply entrenched delusions that she was the target of a conspiracy involving her former employer, with whom she was in dispute over a worker's compensation claim, the police and a number of political figures.
These persecutory delusions had, to some degree, been transferred to her son, the court heard.
In the year before Detective Inspector Anderson's death Fiona had been scheduled by police under mental health legislation, which fuelled her conspiracy theory.
When police responded to a neighbour's complaint on December 6, Fiona and Mitchell declared they were refugees and had the right to defend their property, which they had booby-trapped.
Mitchell stabbed Detective Inspector Anderson twice with a large kitchen knife as he and other officers tried to force their way in through the back door following a prolonged stand-off.
"None of this would have happened but for her chronic descent into mental illness," Mr Brady said.
Fiona was initially charged with murder and once in custody was forced to take anti-psychotic drugs against her will.
The drugs have improved her mental state and she has been compliant in taking her medication and undergoing other psychiatric treatment for at least 12 months, Mr Brady said.
On the first day of their trial earlier this month, the Senior Crown Prosecutor, Mark Tedeschi, QC, allowed Fiona to plead guilty to the lesser charge of manslaughter on the grounds that she was "suffering a substantial impairment due to an abnormality of mind".
On Tuesday he told Justice Robert Allan Hulme that Fiona "bears significant responsibility for her son's act" and her offence is in the highest range of manslaughter.
"In a moral sense she is as much to blame for the death as her son."
Mitchell's barrister, Robert Sutherland, SC, said three forensic psychiatrists opined that his client could have also made a case for a mental illness defence.
But in a "powerful sign of remorse and contrition", Mitchell abandoned plans to argue both a psychiatric defence – which could have led to a not guilty finding - as well as a partial defence of substantial impairment.
Mr Tedeschi said due to Mitchell's mental illness the Crown was not asking for the mandatory life sentence for the murder of a police officer to be invoked.
Justice Hulme will sentence the pair on December 18.
