A woman searched "how to commit murder" five weeks before she used a Nutribullet to blend sedatives into a drink for her partner and gassed him to death in his bed, a jury has heard.

The phrase was among dozens Natasha Beth Darcy allegedly searched online in the months before sheep farmer Mathew John Dunbar died at his property in the regional NSW town of Walcha.

Ms Darcy has been charged with his murder, prosecutors alleging she killed him knowing she would inherit the $3.5m "Pandora" property.

Ms Darcy has pleaded not guilty and contends Mr Dunbar's death was a suicide.

Her trial began in the Supreme Court on Wednesday morning, where prosecutor Brett Hatfield outlined the case against Ms Darcy.

On the evening of August 1, 2017, she allegedly made a pinkish drink for Mr Dunbar using a Magic Bullet blender, that unknown to him contained a cocktail of sedatives.

The Crown alleges she poured it into a glass tumbler and gave it to Mr Dunbar, who drank it, went to bed and stayed there, sedated.

In the early hours of the morning, she is alleged to have placed a plastic bag over his head, affixing it with a white elastic band, and run a laundry hose from the bag to a cylinder of helium gas the pair had picked up in Tamworth earlier that day.

She then turned the gas on, Mr Hatfield told the court.

About 2am, Ms Darcy called triple-0, telling the operator she had found him with the bag over his head.

Paramedics were unable to revive Mr Dunbar.

Mr Hatfield outlined numerous online searches the Crown alleges Ms Darcy made in the months before Mr Dunbar died.

The many search terms he read out in court included "poisonous fungi Australia", "drugs used in suicide", "after suicide is there a crime scene", "murder by inducing heart attack", and "how to commit murder".

On April 18, she searched "how to commit suicide" and "food processor recipes", Mr Hatfield said.

She also allegedly searched "can police see websites you visit on your mobile" and similar phrases on occasion.

In interviews with police she denied making many of the searches, saying it was Mr Dunbar, the court heard.

It was common ground that Mr Dunbar had depression, Mr Hatfield said, and the Crown case is that Ms Darcy "exploited" this fact and killed him in the way she did to "to make it look like suicide".

Ms Darcy's barrister will deliver a short opening address on Wednesday afternoon.

Originally published as Woman's horror act with Nutribullet: court