Natasha Darcy sentenced to 40 years for murder of Mathew Dunbar at Walcha farm

Natasha Darcy was convicted in June last year of murdering Mathew Dunbar in 2017. (Facebook)

A woman who murdered her partner while she stood to inherit his multi-million-dollar property has been sentenced to 40 years in prison.

Natasha Beth Darcy, 46, was convicted in June of murdering Mathew Dunbar in the early hours of August 2, 2017, at his farm at Walcha, in the NSW New England region.

The Crown case was that she sedated the victim with a cocktail of drugs, including an animal sedative, before suffocating him with helium she fed into a plastic bag.

In the Supreme Court in Sydney, Justice Julia Lonergan said she was not persuaded the offence warranted a life sentence.

But she described Darcy as "callous, relentless, and heartless" in her efforts to kill Mr Dunbar.

"The offender has shown nothing remotely resembling remorse or contrition," Justice Lonergan said.

'How to commit murder'

The jury was shown a large number of suspicious internet searches from Darcy's electronic devices, which included: "99 undetectable poisons"; "how to commit murder", and "can police see past web history".

Darcy argued her partner took his own life, and pleaded guilty to assisted suicide, which the Crown rejected.

During the trial, the jury was shown a text message from Mr Dunbar's phone to Darcy's former husband and local paramedic, Colin Crossman, on the morning of the murder.

Prosecutors argued she sent the message herself as a fake suicide note.

But the court heard on the day before the murder, while the couple was at a cafe in Tamworth, Darcy continued to search for advice on the murder method while her partner used his phone to update friends about an earlier medical appointment.

Justice Lonergan said the narrative that Mr Dunbar was troubled and took his own life was "an elaborate, clumsy, and ugly ruse".

A jury rejected Darcy's denials and her argument that Mr Dunbar was depressed and took his own life. (Facebook)

In 2015, Darcy encouraged her partner to make her the sole beneficiary of his estate in the event of his death.

"Don't forget you need to change your will," she wrote to him in one text message.

Justice Lonergan today described Mathew Dunbar as kind and "generous to a fault".

"He wanted to share his love and good fortune with a partner who he could provide for, care for, and cherish," Justice Lonergan said.

Darcy will be eligible to be considered for parole in 2047.