Natasha Beth Darcy sedated and gassed her grazier partner Mathew Dunbar who died in bed on his Pandora property in the Northern Tablelands town of Walcha on August 2, 2017.

Crown prosecutor Brett Hatfield told her sentence hearing on Friday in the NSW Supreme Court that she should be jailed for "life" given Darcy had entirely exploited and manipulated the relationship for financial gain.

Her "determined ugly persistence and planning" was shown in her obsessive internet searches on various methods of killing a person, which could be traced back to February, he said.

These internet queries began with spider venom, poisonous mushrooms, and "toxic wild plants that look like food", and progressed to gas bottles, various sedatives and detailed videos outlining methods.

Articles headlined "science of getting away with murder" and "can police see past web search history", were also looked up and deleted, but later recovered by police.

She left a "false trail' belying his mental ill-health, and made "two dry runs," in either practised or real attempts at his life, Mr Hatfield said.

In June Mr Dunbar was seen by police to be slurring his words, unsteady on his feet, with a very dry mouth and extremely pale, consistent with a drug overdose by Darcy, Mr Hatfield said.

She also sent a fake text message from his phone to her's saying "I am sorry beautiful lady for everything I put you through you deserve better than this."

Then in July after a series of internet searches on injecting muscles, and specifically the calf muscle, Mr Dunbar sustained a calf injury that potentially required surgery.

Defence lawyer Janet Manuell SC argued while these events were suspicious, they could not be proven beyond a reasonable doubt.

Justice Julia Lonergan noted there was a disturbing element to the idea of Darcy sitting down to lunch with the man she lived with while "watching videos on how to complete (his) execution".

A victim impact statement by Mr Dunbar's mother described how her kind-hearted son had hoped to turn Darcy's life around.

"Mathew knew she had troubles and hoped to change her," she said.

"He did not deserve this ... I miss him so much, he was so loving and caring, you would not find another being like him.

"Mathew was my only child and now I am on my own, I have lost my rock and my life won't be the same again."

An expert report tendered to the court found Darcy had a complex personality structure with anxiety disorder.

Ms Manuell said she could potentially be seen to have an obsessive quality too, given the striking amount of murderous internet searches she made.

"It's very hard to explain why somebody would make so many searches, accepting that Ms Darcy had settled on a particular course of action," she said.

Darcy's risk to future partners was also aired following past attempts on estranged husband, paramedic Colin Crossman.

In 2009, she hit him on the head with a hammer as he slept and days later sedated him and burnt down their house as he slept.

Darcy met Mr Dunbar on a dating website in 2014, but the Crown argues it didn't take long for her to start pushing for a change to his will so she would inherit his $3.5 million property.

Darcy is due to be sentenced on December 3.

Australian Associated Press