A woman accused of giving her sheep farmer boyfriend a cocktail of sedatives blended in a Nutribullet before gassing him to death with helium has pleaded not guilty to murder.

Natasha Beth Darcy, 44, is accused of murdering her 42-year-old boyfriend Matthew Dunbar at his home in Walcha, New South Wales on August 2, 2017.

It is alleged Ms Darcy gave her boyfriend the blitzed sedatives before wrapping a plastic bag around his neck and placing a tube connected to a helium tank underneath.

She appeared on a video screen in the NSW Supreme Court on Friday and pleaded not guilty.

Natasha Beth Darcy (right), 44, is accused of murdering her boyfriend Matthew Dunbar (left), 42

The Crown argued Ms Darcy searched on the internet: 'Does helium show up in an autopsy,' 'Suicide bag,' 'Euthanasia device', and 'Helium filled exit bag.'

The prosecution accused Ms Darcy of using a fake name when she purchased the ram sedatives used to drug Mr Dunbar.

Ms Darcy's lawyer argued her boyfriend had committed suicide.

It is alleged Ms Darcy gave her boyfriend a cocktail of sedatives before wrapping a plastic bag around his neck and placing a tube connected to a helium tank underneath on August 2, 2017

After allegedly threatening to end his life, Mr Dunbar was admitted to a mental health facility two months prior to his death, the lawyer said.

Mr Dunbar's mother, Janet, said she still struggles to comprehend why anyone would want to murder her son.

'He'd give you the shirt off his own back and unfortunately he was too trusting,' Ms Dunbar told 9News.

'He just wanted to belong to a family. Family meant everything to him,' she said.