Natasha Beth Darcy had "quite a substantial motive" to murder him as she was due to inherit his $3.5 million property despite lying to police about her knowledge of his will, prosecutor Brett Hatfield said on Wednesday.

He is giving the crown closing address at the 46-year-old's NSW Supreme Court murder trial which began on March 31.

She has denied sedating and gassing 42-year-old Mathew Dunbar, who was found dead on his Pandora property in the Northern Tablelands town of Walcha on August 2, 2017.

She contends he killed himself, but the Crown has rejected her guilty plea to aiding or abetting suicide.

"You will be safely satisfied that he had no intention to kill himself that night," Mr Hatfield said on Wednesday.

He alleged that at some time after dinner Darcy sedated Mr Dunbar using a Nutribullet to blend a cocktail of sedatives, before moving a gas tank into his room and gassing him in his bed.

She had employed a method used by "Betty" in an instructional video shown to the jury which the Crown alleged she watched days before his death.

It showed an elderly woman with a large name tag reading "Betty" demonstrating a suicide method while high energy music played in the background.

Mr Hatfield referred to the evidence of a former friend of Darcy's who was offered $20,000 "to tell a number of quite specific lies about Mr Dunbar to assist the accused in this murder trial".

Darcy allegedly said once she was found not guilty she would automatically inherit the property and would be willing to give the woman $20,000.

After alleged requests by Darcy, Mr Dunbar had changed his will in May 2015, making her the sole inheritor.

At the time the property was valued at between $3.4 to $3.6 million, but Mr Hatfield said it was eventually sold for $4.6 million.

In police interviews she said she had no knowledge of Mr Dunbar altering his will, before eventually she admitted knowing it had been changed but thought it had not activated by his solicitor.

The prosecutor said her denials were deliberate lies, alleging her clear motive in murdering her partner was to obtain the inheritance.

"His death was something she had been planning ... for most of the preceding year," he said.

He listed numerous iPhone searches, said to have been carried out by Darcy, dating back to February 2017 after she moved in with Mr Dunbar in October 2016.

Topics included redback spiders and mushrooms, the latter not being "the history of a mushroom forager interested in eating them but their capacity to cause the death of someone".

He is continuing his address.

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Australian Associated Press