But Barbara Eckersley's barrister insists the 69-year-old did not intend to kill Dr Mary White, and says the 92-year-old may have died "independently" of his client's actions.

The trial of Mrs Eckersley, who has pleaded not guilty to murder over the 2018 death of her mother, began in the NSW Supreme Court at Goulburn on Wednesday.

In his opening address, Crown prosecutor Paul Kerr told the jury it would be "strange" if they did not feel some sympathy for Mrs Eckersley when they heard how she had cared for her ailing mother.

He said murder was usually committed, for reasons such as revenge and jealousy, by "not very nice people".

"That is not Barbara Eckersley," he said.

Mr Kerr said Mrs Eckersley had been, by all accounts, a "loving, caring and compassionate" daughter.

"What she did was most likely done with love and compassion, and of course pain and sorrow," he said.

But Mr Kerr said Mrs Eckersley had intentionally and unlawfully taken her mother's life in a way that made her guilty of murder.

The prosecutor told the jury that Mrs Eckersley and her husband Richard had cared for Dr White when she came to live with them in Bundanoon around 2014.

He said Dr White, who had been diagnosed with vascular dementia, later moved to the Warrigal aged care facility in the Southern Highlands town after a major stroke in 2016.

There, she was "pretty much confined to the bed", unable to communicate and paralysed on one side of her body.

Mr Kerr told the jury that Mrs Eckersley experienced "conflict" with staff at the facility about how they were caring for Dr White.

A decision was made to move Dr White to a facility in Coffs Harbour, but she died two days before the transfer was to occur.

Mr Kerr told the jury Dr White "took a turn" on August 5, 2018, after Mrs Eckersley mixed pentobarbital, also known as "green dream", into her dinner and fed it to her.

He said that a few days on, Mrs Eckersley went to police and told them: "I helped my mother die. I put barbiturates into her food ... I contributed to my mother's death."

He said Mrs Eckersley had obtained the "green dream" some 20 years earlier, when she worked as a wildlife carer in Canberra and used it to euthanise animals.

Mr Kerr said it was unclear how much of the drug had been mixed into Dr White's meal, but a "toxic to fatal" amount of it was found in her system.

While Mr Kerr alleged Mrs Eckersley laced the food to "help [her] mother die", he said she had also told police she had just been trying to ensure Dr White was not in pain or distress during the planned journey to Coffs Harbour.

Defence barrister Kieran Ginges, in brief opening remarks, said the jury would hear about "the close and caring bond [Mrs Eckersley] had with her mother".

He said Mrs Eckersley had not intended to kill Dr White, and experts would cast doubt on whether the "green dream" had in fact made a "substantial or significant" contribution to the scientist's death.

Mr Ginges said the 92-year-old "could have died at any time" as a result of numerous health conditions.

The jury was also told Mrs Eckersley's defence would claim she was "substantially impaired" at the time in question by a major depressive disorder.

The court heard later on Wednesday from members of Mrs Eckersley's family, including her husband.

Richard Eckersley said recalled the moment his wife first told him she had "given her mother something".

"It was the shock of my life," he told the court.

He also said he, "like most Australians", supported the legalisation of voluntary euthanasia, and he believed his wife did as well.

Justice Robert Beech-Jones stressed to the jury that it was "entirely legitimate" to hold such opinions, and that Mrs Eckersley was "not on trial" for any views she might have on that topic.

The trial continues.

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