Natasha Beth Darcy pleaded guilty to aiding and abetting suicide, but prosecutor Brett Hatfield told the jury the Crown rejected the plea and contended she killed Mathew Dunbar.

The 46-year-old has pleaded not guilty in the NSW Supreme Court in Sydney to murdering the 42-year-old sheep farmer on his property Pandora in the Northern Tablelands town of Walcha in the early hours of August 2, 2017.

He was found in his bed with a plastic bag over his head, in which was inserted a tube connected to a gas cylinder.

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In the crown opening address on Wednesday, Mr Hatfield said the property was valued at $3.5 million and in 2015 Mr Dunbar made a new will leaving all of his estate to Darcy.

He alleged this was Darcy's primary motive to murder her partner by faking his suicide.

Much of the crown case centred of many internet searches made over a period of months on various phones and a computer, searches which Mr Hatfield alleged were made by Darcy.

"The accused acknowledges she made some but not all the searches," he said.

"She has suggested the deceased made a number of them."

Search topics included poisonous spiders and mushrooms, toxic plants, suicide methods, how to commit murder and "can police see websites you visit on your mobile".

After the couple had dinner and shared red wine, Darcy allegedly mixed a number of sedatives in a blender, which she gave to Mr Dunbar in a drink in a glass tumbler.

Darcy was married to paramedic Colin Crossman, but they had been separated for some time.

At 1.14 am on August 2, Mr Crossman received a text from Mr Dunbar's phone which said "tell police to come to house, I don't want Tash or kids to find me".

Darcy rang triple zero at 2am saying she had discovered her partner in his bed with a plastic bag over his head and gas or something going into it.

Mr Hatfield said the Crown would rely on evidence from Mr Crossman to "prove a number of tendencies on behalf of the accused to act in a particular way".

He referred to a January 2009 episode when she asked her husband how hard a person would have to be struck in order to cause damage to the temple.

During the night she obtained a hammer and at about 4am when her husband was sleeping, she struck him across his temple.

She alleged he had been attacked by an intruder but later admitted being the culprit, the prosecutor said.

Mr Hatfield said on another occasion when her husband was asleep she took a tin of petrol from the garage and poured it on the bedroom floor and set it alight.

She had earlier given him a meal of tacos and samples later showed he had sedatives in his system, drugs he said he did not knowingly take.

"There will also be evidence about financial circumstances and the insurances at the relevant time," he said.

Darcy's barrister will give the defence opening later on Wednesday.

Australian Associated Press

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