Prosecutors call for life prison sentence for Natasha Beth Darcy after calculated murder of Walcha sheep grazier Mathew Dunbar

Natasha Beth Darcy, 46, was visibly emotional as she fronted a sentencing hearing in the NSW Supreme Court on Friday, after she blended a cocktail of drugs and fed it to her partner Mathew Dunbar before he took his last breath on August 2, 2017.

Crown prosecutor Brett Hatfield told the court Darcy's culpability was "so extreme" because she researched the murder for six months and was backed by a purely financial motive.

"Very shortly after she commenced living with the deceased ... she began researching her plan to murder the deceased in order to inherit his assets," he said.

Mr Dunbar's death left behind a trail of heartbreak as the court heard a victim impact statement from his estranged mother Janet, as part of the Crown's submissions.

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"Mathew did not deserve this," she said.

"I miss my son very much, he was so loving and caring, you will not find another human being like him.

"I have lost my rock and my life won't be the same again."

Mr Hatfield said Darcy exploited her relationship with Mr Dunbar, manipulated him, staged a suicide, lied to police, left a false trail, researched how to get away with murder and tried to conceal her internet history.

He submitted Darcy, dubbed the 'widow of Walcha', had practiced ways to kill Mr Dunbar in two "dry runs" where she allegedly drugged him on June 21 and injected a ram sedative into his calf in early July.

Mr Hatfield said Darcy had carried out specific internet searches and the dry runs showed her planning and persistence in wanting to kill Mr Dunbar.

Medical experts in Darcy's 10-week murder trial couldn't confirm what may have been in Mr Dunbar's system at the time, or whether Darcy had inflicted his "problematic" leg injury.

Justice Julia Lonergan said any finding would be based on circumstantial evidence and that's where the alleged dry runs become "messy" and "loaded" because the jury hadn't determined them.

She said the Crown's life sentence argument may still stand if she couldn't find that Darcy had carried out the alleged dry runs.

Defence counsel Janet Manuell submitted that although "suspicious", there was simply not enough evidence to satisfy the court beyond reasonable doubt the incidents had occurred.

Justice Lonergan said the dry runs would make "so much sense" but there was nothing to prove for a fact that Darcy had done anything.

"Her persistence is remarkable," she said.

The court heard Darcy maintains she is innocent of murder despite the jury verdict in June, but Ms Manuell said that wasn't uncommon in these types of cases.

She argued there was hope that Darcy could change her behaviour and insight in prison, and become a different person with time and treatment for anxiety and personality disorders, if the court decided against a life sentence.

"There will be a lengthy sentence imposed, there's no question about that," she said.

"Who knows what will happen in the decades to come?"

Ms Manuell admitted Darcy's internet searches in the lead up to Mr Dunbar's murder were striking because they were "obsessive" and "discomforting".

Justice Lonergan asked whether anxiety could be a problem for Darcy if she found another vulnerable partner in the future.

Defence and Crown submissions and other documents had already been handed up to Justice Lonergan to consider in her sentencing.

Darcy is expected to be sentenced in early December.

She was found guilty of murder after a jury of 11 accepted the Crown case at the end of the 10-week trial in June.

The jury decided Darcy had the motive and means to murder Mr Dunbar, exploiting his generosity to ensure she would solely inherit his multi-million dollar farm.

Darcy's web of lies was exposed during the trial, which heard from dozens of witnesses including friends, family, medical experts and police officers.

The jury took less than three days to reach its guilty verdict, finding she sedated the sheep grazier on animal sedatives and a cocktail of drugs blended in a Nutribullet, before gassing him with helium in his bedroom at his property on the outskirts of Walcha, staging it as a suicide.

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