Natasha Beth Darcy sentenced in NSW Supreme Court for murder of Walcha sheep grazier Mathew Dunbar in 2017

But, for his loved ones, the pain of the loss will last a lifetime.

Speaking moments after the sentence was handed down, the victim's estranged mother Janet Dunbar described her son as a "giver" and "an absolutely beautiful human being".

She said the sentence brought little relief.

"You don't get closure, you just learn to live with it," Ms Dunbar said. "I wish I had more time with him."

Darcy, 46, showed no emotion as she was ordered to remain behind bars until at least 2047, when she's eligible for parole, in the NSW Supreme Court on Monday.

Darcy was found guilty of murder in June last year after a 10-week trial. The jury found she sedated her 42-year-old partner with a cocktail of drugs blended in a magic bullet before she gassed him in his bed at his property Pandora, on the outskirts of Walcha, on August 2, 2017.

Justice Julia Lonergan told the court during sentencing that Darcy, who appeared by video-link had been "callous, relentless and heartless" in her efforts to "get rid of" Mr Dunbar for his money.

She said Darcy had manipulated Mr Dunbar into making her the sole beneficiary of his will, which included his Pandora property, worth about $3.4 million at the time.

Darcy's search history revealed she had spent months before Mr Dunbar's death trawling the internet as she plotted the murder and began to lay a false trail of evidence, Justice Lonergan said.

She said Darcy had staged the murder scene at Pandora to look like Mr Dunbar had killed himself.

When Mr Dunbar was likely already dead, Darcy sent a "fake suicide" text off his phone to her estranged husband and Walcha paramedic Colin Crossman.

Justice Lonergan said although Darcy was sadly successful in killing Mr Dunbar, her plan failed to outsmart detectives.

"Her lies and methods were stupid, clumsy, and ugly ... they were not however good enough to evade detection," she said.

Even in her police interviews, she said Darcy had told lies about Mr Dunbar's health, sexuality and financial position to try to deceive the investigating officers.

Justice Lonergan read through an extensive history of Google searches on phones and a computer, starting in February 2017, as part of the sentencing proceedings.

"This murder involved a very high degree of planning for well over three months," she said.

Prolific searches included "how to commit murder" and "drugs used in suicide" before she came across a website about helium asphyxiation in late July 2017.

The court heard Darcy then began searching for specific information, items and techniques to carry out her plan successfully, including looking for helium tank hire stores in Tamworth.

She took to Google again to try and cover up her own research, asking "can police see past web history".

Justice Lonergan made factual findings on Monday that Darcy had drugged Mr Dunbar on one occasion in June 2017 and had also injected his leg with ram sedative before the murder, causing a calf condition.

"It is conduct relevant to the extent of planning and persistence in relation to the murder she successfully completed," she said.

"Emotional abuse and sneaky physical attacks escalated into a more focused and fool-proof method to achieve the outcome she desired."

The court heard Darcy continued to refuse to take responsibility for sedating and gassing Mr Dunbar to death almost five years on, despite the jury verdict, and had maintained her own fictitious account.

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

She listed the aggravating factors of the murder including that Mr Dunbar was killed in his own home, Darcy was on parole at the time and that the motive was purely financial.

She said Darcy was not a person of good character and had, in 2009, hit her estranged husband Mr Crossman in the head with a hammer while he slept before setting the house on fire for the insurance money.

Darcy first sparked a relationship online with Mr Dunbar in 2014.

Justice Lonergan said the loss of Mr Dunbar was significant to Walcha and extended her condolences to friends and loved ones.

She said the murder was one of "high objective seriousness" but said a "very lengthy, fixed sentence" was appropriate, rather than a life sentence.

Darcy will be 77 years old when she is eligible for release.